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taurochohc acid completely precipitates native proteids, but not album-
oses or peptones.''' This in part explains the precipitation observed when
a solution in which peptic digestion is going on (or gastric chyme) is
mixed with bile ; but part of the precipitate is doubtless mucin from
the bile itself. The subject has been investigated by Hammarsten,^
who found that syntonin was completely, peptone only partially, pre-
cipitated from an acid solution in which peptic digestion of hard-boiled
white of egg had been carried out, by the addition of bile from which
the mucin had been removed by alcohol. Hammarsten supposes that
the purpose of this precipitation of the semi-digested proteid, which
must occur in natural digestion when the gastric chyme comes in
contact with the bile, is that it may, by adhering to the intestinal wall,
be longer subjected to intestinal digestion than it would be if it remained
in solution.

^ Centralhl. f. d. micd. JFisscnscJi., Berlin, 1885, Rd. xxiii. S. 121. Similar results have
been obtained by Prevost and Binet, Coonpt. rend. Acad. d. sc, Paris, 1888, tome cvi. p.
1690 ; Winteler, Inaug. Diss., Dorpat, 1892.

" Kunkel, Arch. f. d. ges. Physiol., Bonn, 1877, Bd. xiv. S. 344 ; Spiro, Arch. f. Anat.
u. Physiol., Leipzig', 1880, Supp. Bd. S. 50.

^ Sitzunr/sb. d. Tc. Akad. d. TVissensch. , Wien, 1878, Bd. Ixxvii. Abth. 3.

■* "Die Verdauungssafte, " S. 217. ^ Vide "Fat Absorption," p. 454.

^ Neumeister, " Lelirbuch d. physiol. Chem.," Jena, 1893.

■? Monatsh.f. Chem., Wien, 1883, Bd. iv. S. 89 ; 1885, Bd. vi. S. 95.

^ Jahrcsh. ii. d. Fortschr. d. ges. Med., Erlangen, 1870, Bd. 1, S. 106. See also
Chittenden and Cummins, Am. Chem. Journ., Baltimore, 1885, vol. vii. p. 36 ; Jahresh.
ii. d. Fortschr. d. Thier-Chem., Wiesbaden, 1885, Bd. xv. S. 319.



DIGESTION OF CARBOHYDRATES. 393



DIGESTION OF CAEBOHYDRATES.

The digestion of carbohydrates is brought aljout Ijy the action of two
distinct classes of enzymes, namely — 1. Those which act on starches,
producing sugars and dextrins : these are called amylolytic or diastatic
ferments. 2. Those which act on various saccharoses, producing glu-
coses : these are called inverting ferments.

The two chief amylolytic ferments found in the digestive juices are
ptyahn and amylopsin. The action of these ferments on starch may be
demonstrated by adding to starch paste, either saliva or pancreatic juice,
or a watery infusion of salivary or pancreatic gland. The paste very
soon becomes quite fluid, and if the fluid be tested chemically for starch,
it will be found that this substance is rapidly disappearing, and that a
reducing material is being formed in continuously increasing amount in
the solution. This testing may be done by removing a drop of the
solution at intervals, and mixing it with a drop of a solution of iodine.
At first the deep blue colour given by starch is obtained ; this is replaced
after a time by a violet, this again by a red colour, and finally no
coloration at all is obtained. If at each of these stages portions of the
solution be tested with Fehling's solution, it will be found that it has
acquired reducing power, and that the amount of reduction increases
with the length of time during which the action goes on.

The diastase of malt is very similar in its action to both these
ferments, but is not identical with either of them, as is shown by the
fact that while ptyalin and amylopsin act best at body temperature, the
optimum temperature for the action of malt diastase is about 55° C.

Products of digestion of starch. — Whether the ferments are
identical or not, their action, according to all observers, is the same. It
was shown by Leube,^ in 1831, that saliva dissolves starch-paste and
forms sugar, and the same was shown for pancreatic juice by Bouchardat
and Sandras,^ in 1845. It was for many years believed that the
action of these ferments was closely analogous to that of mineral
acids, and that the sugar produced was grape-sugar. Dextrin was
supposed to be the first stage in the process of saccharification, and
from the dextrin it was thought that grape-sugar w^as afterwards
formed. Musculus ^ was the first to show that all the starch was not
so converted into sugar, but that saccharification only proceeded until
the solution gave no longer a colour reaction with iodine ; on adding
fresh starch-paste, the reaction recommenced and proceeded as before,
until again all colour reaction with iodine had vanished, when, as
before, the reaction slackened and stopped, although there remained
plenty of dextrin in the solution.

According to the earHer work of Musculus, the quantitative relationship in
which the sugar and dextrin stand at the end of the reaction is, one part of
sugar to two of dextrin ; his later papers gave the reaction as stationary,
when approximately equal quantities of sugar and dextrin are present in the
solution.*

^ Arch. f. d. ges. Naturl., Niirnberg, 18-31.
" Conipt. rend. Acad. d. sc, Paris, 1845, tome xx. p. 1085.
^ Journ. de pharm. ct chim., Paris, 1860, S^r. .3, tome xxxvii. p. 419.
•i Payen, Chera. Centr.-Bl., Leipzig, ]865, S. 845; Schwarzer, iUd., 1870, S. 295;
Schulze u. Marker, ibid., 1872, S. 823.



394 CHEMISTRY OF THE DIGESTIVE PROCESSES.

Still later, Sheridan Lea,^ working with much more dilute solutions than
were usually employed by other experimenters (0*4 to 4 per cent.), found as
much as 85 per cent, of the starch converted into sugar ; and by more closely
approximating the conditions of experiment to those of natural digestion, by
carrying out the experiment in a dialyser instead of in a glass vessel, obtained
a still greater reduction in the percentage of dextrin formed (7 to 8 per cent.).
He is therefore of the opinion that in the alimentary canal starch is
completely converted into sugar before absorption. The increase in sugar
formation, due to removal of the products of digestion, was more marked in
working Avith strong than in working with dilute solutions ; this also goes to
show that it is the accumulation of maltose in the solution which slackens and
stops the reaction. All chemical reactions involving hydration, such as saponi-
fication of esters, become stationary at . a determinate point when a fixed
proportion of hydration has taken place ; and this point is rigorously de-
termined by the concentration in the solution of the various factors in the
reaction. If the substance or substances formed in the reaction be continuously
removed from the solution, or changed in nature as they are formed, the
reaction proceeds to completion ; but if the products of the reaction remain in
solution unchanged, at a perfectly fixed point, dependent on the concentration
in solution of each of the reacting substances, equilibrium is established, and
no further change in the composition of the solution takes place.

On the other hand, Musculus and Gruber ^ claim to have isolated a dextrin
after acting on starch paste with diastase for five days, by precipitating the
dextrin with alcohol ; on this dextrin, diastase, even in the absence of the sugar,
has no further action.

In 1872, O'SulKvan ^ rediscovered the sugar described by Dubrunfaut *
as formed by the action of malt extract on starch paste, isolated it,
investigated its properties, and named it maltose. When it was so shown
that the sugar formed by the action of malt diastase is not grape-sugar,
attention was directed naturally to the sugars similarly formed by the
action of the digestive enzymes of the sahva and pancreatic juice.
^asse^ stated that the sugar formed by the action of saliva is not
dextrose, but another sugar of different reducing power, to which he
gave the name of 2Jt'i/alose, which, however, was not maltose, as its
reducing power was doubled on boihng with acids, while that of maltose
was only increased by one-half. Soon after, v. Mering and Musculus ^
conclusively showed that the sugar really formed both by ptyalin and
amylopsin is identical with O'SuUivan's maltose, and these results have
been abundantly confirmed by subsequent observers. This result they
established by the amount of increase of reducing power and reduction
of rotating power, following boiling with a dilute mineral acid, as well as
by the observation of birotation in the solution after boiling, which
could only be due to the formation of grape-sugar. Such an analysis is
rendered easy by the widely different specific rotatory powers and
reducing powers of the two sugars (see table, p. 396).

The action of malt diastase, ptyalin, or amylopsin on starch paste
takes place in several stages, corresponding to which more or less

1 Journ. PIl'ijswI., Cambridge and London, 1890, vol. xi. p. 226.

'^ Zlschr. f. 'physiol. Chem., Strassluu'g, 1878, Bd. ii. S. 187.

^ Journ. Chem. ,S'oc., London, 1872, vol. xxv. p. 579.

■* Ann. ch chini. etphys., Paris, 1847, tome xxi. p. 178.

" Arch. f. d. ges. Physiol., Bonn, 1877, Bd. xiv. S. 477 ; see also Seegen, Ccntralhl. f. d.
med. JVissensch., Beriin, 1876, S. 8.'il.

« Ztschr. f. physiol. Chem., Strasslmrg, 1877, Bd. i. S. 395 ; 1878, Bd. ii. S. 403. See
also Brown and Heron, Proc. Pay. Soc. London, 1880, vol. xxx. p. 393.



PR OD UCTS OF DIGESTION OF ST A RCH. 395

well-differentiated substances have been described. The first step in
the action is, according to all observers/ the formation of soluUe starch
(amigdulin, amidulin, amidogen, or amylodextrin). This substance is
rapidly formed, usually in one or two minutes ; it gives the same blue
reaction with iodine as raw starch or starch paste, and is precipitated by
tannic acid, by which it is distinguished from the dextrins formed in the
subsequent stages.

In the second stage, this soluble starch is decomposed into a sub-
stance giving a red colour with iodine and maltose. The substance
giving the red colour now gets the name given by Briicke of erythro-
dextrin; it corresponds to Nasse's dextrin, Griessmayer's dextrin-1
and Bondonneau's dextrin-a.

In the third stage, this erythrodextrin is split up into a dextrin (or
several dextrins), giving no coloration with iodine, and hence called
achroodextrin, and a further quantity of maltose. Finally, according to
some, a part of this achroodextrin breaks up, yielding more maltose,
and a variety of achroodextrin, altogether unaffected by diastatic
ferments, which with the maltose split off at different stages from the
intermediate products, forms the final product of the reaction, no matter
how long prolonged.

These successive changes may be represented as in the following
scheme :—

Starch

I
Soluble Starch



Erythrodextrin Maltose



Achroodextrin Maltose



All observers are agreed as to the existence of soluble starch, and
practically all as to that of erythrodextrin, although Muscnlus and Meyer ^
state that, on carefully mixing dextrin stained with iodine, with soluble
starch stained with iodine, they obtained the colour of erythrodextrin, and
conclude that what has been called erythrodextrin is probably such a mixed
colour. This result has not been confirmed by other observers ; still it
should be borne in mind that a pure substance has not yet been isolated, and
that at present erythrodextrin is only a name given to a substance supiMsed
to exist, because of a red colour which is obtained at a certain stage in the
digestion of starch by diastatic enzymes. The material which is found later
in the process, which is not a sugar and gives no coloration with iodine, has
been called achroodextrin, but it has none of the constant properties of a
pure simple substance, and is probably a mixture of several substances
(achroddextrins), though as yet none of these have been properly isolated.

Musculus and Gruber,^ working on starch solutions with dilute acids and
with diastase, differentiated, according to varying conditions of temperature,
amount of diastase added, and length of time of action of the ferment, three
achroodextrins (a, y8, and y), possessing, according to these observers, different

^ Nasse, Arch. f. d. ges. Physiol., Bonn, 1877, Bd. xiv. S. 474 ; Griessmayer, Chevi.
Centr.-BL, Leipzig, 1871, S. 636 ; Briicke, " Vorlesungen," and Sitzungsb. d. k. Akad. d.
Wissensch., Wien, 1872, Abth. 3; Bondonneau, Compt. rend. Acad. d. Sc, Paris, 1875,
tome Ixxxi. pp. 972, 1210; Maisculus, Ztschr. f. physiol. Chem., Strassburg, 1878, Ijd. ii.
S. 177.

"^ Ztschr. f. physiol. Chem., Strassburg, 1880, Bd. iv. S. 451.

^ Ibid., 1878, Bd. ii. S. 177.



396



CHEMISTR V OF THE DIGESTIVE PROCESSES.



specific rotatory powers and reduction coefficients ; but they scarcely give
adequate proofs that they are describing pure substances. ^ They found that
even after tvs^elve months a portion of the dextrin remained unconverted into
maltose, and the substance so remaining was unfermentable by yeast. This
substance is y-achroodextrin, and is formed together with some maltose by the
splitting up of yS-achroddextrin, which in its turn is formed by a similar
decomposition of a-achroodextrin.

The following is a summary of their results: — ■





Sp. Rotatory Power.


Relative reducing

Power for Fehling's

Solution.


Reaction with
Iodine.


Soluble starcli .

Erythrodextrin

a-Acliroodextrin

Maltose .
Grape-sugar








218°

210°
190°
150°
150°
50°


6

12
12
28
66
100


Reddish blue.

Red.

No coloration.

J)



The digestion of starch by diastatic enzymes consists of a breaking
up, through several more or less well-defined stages, by a process of
gradual hydrolysis, of a very complex molecule into a much simpler one,
and might be represented schematically by the following general
equation, which cannot be made more definite, because we are un-
acquainted with the molecular weights of starch and dextrin, only
knowing that they are very large —



Starch



Maltose



Dextrin



(CeH.,A)n + (H,0),



^XC,H,,0„H,0) + ^^\CeH, A)p



That is, starch and water, in presence of a suitable ferment, yield
maltose and different dextrins, but we are ignorant of the value of
n, m, and p.

Attempts to carry too far the analogy between the action of
dilute mineral acids and that of the diastatic ferments on starch,
led, as already stated, to an error, which persisted for several years,
as to the products of the latter action. Nevertheless, a close analogy
does exist between the two processes; both are essentially hydration
processes ; and in both the same stages may be observed. They
only differ in two respects, first, that the dilute acid at boiling tem-
perature acts much more rapidly; secondly, that it proceeds a
stage further, and very rapidly converts the maltose formed into grape-
sugar.

These successive changes may be best observed by boiling with very
dilute acid (-2 per cent, or less). Soluble starch is first formed, giving, on
neutralisation, a blue with iodine ; next, is an intermediate stage, in which a
violet is obtained followed by a stage giving a red colour (erythrodextrin) ;
and finally a stage is reached at which a coloration is no longer obtained



^ According to Brown and Morris (see Trans. Chein. Soc, London, 1885, p. 527 ; 1889,
p. 462), the chemical and physical properties of these different achroiidextiins might be
given Ijy a variable mixture of one achroijdextrin possessing no reducing power with mal-
tose. They admit the existence besides achroodextrin, of waltodextrm (Herzfeld). a body
intennediate between achroodextrin and maltose, but more nearly allied to the latter.



PR on UCTS OF DIGESTION OF STAR CII. 397

(achroodextrin). If, when this stage is reached, the sohition is rapidly
cooled and neutralised, a little maltose can be separated from accompany-
ing dextrose, showing that maltose is here also formed, hut is con-
verted rapidly into grape-sugar.

Although maltose is the chief sugar formed in the action of Ijoth
ptyalin and amylopsin upon starch, yet a trace of grape-sugar is also
formed.^ The quantity of grape-sugar formed is in both cases small, but
is greater in the case of the pancreatic ferment. It has recently been
discovered that in both salivary and pancreatic digestion, besides
maltose and small quantities of grape-sugar, another sugar, isomaltose,^
is formed, in considerable quantity. The relative quantity of the three
sugars varies with the quantity of ferment present, and the duration of
the experiment. A weak ferment and short time of action favour the
formation of isomaltose; by much ferment and prolonged action large
quantities of maltose are produced, accompanied by traces of dextrose.^
It is stated that traces of an inverting ferment are present, both in the
salivary and pancreatic glands, especially the latter ; and it is possible
that the traces of dextrose formed may be due to the action of these on
the maltose and isomaltose first formed.

The action of the amylolytic enzymes on glycogen is precisely similar
to their action on starch ; dextrin, maltose, and isomaltose being formed
in very much the same proportions.*

The production of maltose by the diastatic ferments is not the end
of the digestion of amyloses; there is evidence that maltose never
reaches the systemic circulation. If it be injected intravenously it is soon
discoverable in the urine ; ^ this shows that in digestion it is inverted,
either before it is absorbed, or after absorption and before reaching the
systemic circulation.

This further process of hydrolysis may be to some extent carried out,
so far at least as concerns that portion of maltose arising from
salivary digestion, by the hydrochloric acid of the gastric secretion, but it
is mainly brought about by an inverting ferment, discovered by Brown
and Heron in the mucous membrane of the small intestine, and also in
the succus entericus.*^

The succus entericus (as well as the intestinal mucous membrane and
glycerin or water extracts of it) possesses only a feeble diastatic action

^ Muscuhis and Gruber, Ztschr. f. physiol. Chem., Strassburg, 1878, Bd. ii. S. 177 ;
Museulus and von Mering, ibid., 1878, Bd. ii. S. 403 ; von Mering, ibid., 1881, Bd. v. S.
185 ; Brown and Heron, Ann. d. Chem., Leipzig, 1879, Bd. cxcix. S. 165 ; 1880. Bd. cciv. S.
228 ; Proc. Roy. Soc. London, 1880, p. 393.

2 Kitlz u. Vogel, Ztschr. f. Biol., Miinchen, 1894, Bd. xxxi. S. 108. The existence of
isomaltose is, however, denied by Brown and Morris [Trans. Chem. Soc, Loudon, 1895, vol.
Ixvii. p. 737), who state that it is a mixture of maltose and dextrins.

^ Jourii. Physiol., Cambridge and London, vol. xv. ; Abelous, Compt. rend. Soc. de biol.,
Paris, 1891.

^ Hensen, Verhandl. d. phys.-med. Gesellsch. zu TVurzbu7-g, 1856, Bd. vii. S. 219;
Virchow's ArcMv, 1857, Bd. xi. S. 395 ; Claude Bernard, Gaz. med. de Paris, 1857, No. 13 ;
J. Seegen, Centralbl. f. d. med. Wissensch., Berlin, 1876, S. 849 ; Arch. f. d. ges. Physiol.,
Bonn, 1879, Bd. xix. S. 106; Kitlz u. Vogel, Ztschr. f. Biol., Miinchen, 1894, Bd. xxxi.
S. 108.

5 Bimmerraann, Aoxh. f. d. ges. Physiol., Bonn, 1879, Ed. xx. S. 201 ; Philips,
Jahresb. ii. d. Fortschr. d. Thier-Chem-., Wiesbaden, 1881, Bd. xi. S. 60 ; Dastre at Bour-
quelot, C'om2}t. rend. Acad. d. sc, Paris, 1884, tome xcviii. p. 1604; Bourquelot, Joiorn. de
Vanat. etphysioL, etc., Paris, 1886, tome xxii. p. 161.

^ Brown and Heron, Proc. Roy. Soc. London, 1880, p. 393; Ann. d. Chem., Leipzig,
1880, Bd. cciv. S. 228 ; Vella, Untersuch. z. Naturl. d. Mcnscli. ii. d. Thiere, 1881, Bd,
xiii. S. 40; Bourquelot, Compt. rend. Acad, d, sc, Paris, 1883, tome xcvii. p. 1000.



398 CHEMISTRY OF THE DIGESTIVE PROCESSES.

on starch/ but has a remarkable power in converting maltose into
grape-sugar. Brown and Heron surmised from this that maltose
would be found to be a non-assimilable substance ; unknown to them,
Bimmermann had already shown this to be the case, and many subse-
quent observers have confirmed the result.

Digestion of cane-sugar. — Cane-sugar resembles maltose in not
being directly assimilable from the blood ; after intravenous injection it
is excreted by the kidneys. In the course of digestion it is either com-
pletely inverted while in the alimentary canal, or may in part be so
changed in its passage through the absorbing cells of the mucous
membrane.

Some cane-sugar is inverted in the stomach, probably by the action
of the hydrochloric acid there present.^ Lehmann ^ repeatedly found, in
the stomach and duodenum of rabbits fed on beetroot, invert-sugar only.
Seegen found that, after feeding dogs on cane-sugar, the stomach always
contained a small amount of a reducing sugar along with a great deal of
unchanged cane-sugar ; and that the small intestine contained no sugar.
He argues from this, that all the cane-sugar is inverted in the stomach,
the invert-sugar being there absorbed as fast as it is produced.
It is probable, however, from the work of other observers, that a con-
siderable part of the inversion takes place in the small intestine by the
action of the intestinal juice, and it may be also by the direct action
of the cells of the intestinal mucous membrane.

Watery infusions of the mucous membrane from any part of the
small intestine are capable of inverting cane-sugar,* and the same
power is possessed by the intestinal contents in animals which have
been killed during active digestion.^ The intestinal juice obtained by
Vella ^ from fistulse almost instantly inverted cane-sugar, and a strong
inverting action of pure succus entericus has been observed by many
others.

The inversion of the saccharoses by the intestinal juice is brought
about by enzymic action, but very little is known of the enzyme
or enzymes involved. It was supposed by Hoppe-Seyler and Thier-
f elder '^ that the inversion might be due to bacterial action or to
inverting enzymes taken in with the carbohydrate food, but the former

^ A diastatic action on starch -was found by Schitf, Jaliresb. it. d. Fortschr. d. ges. Med.,
Erlangen, 1867, Bd. i. S. 155 ; Eichhorst, Jahreah. u. d. Fortschr. d. Thier-Chem.,
Wiesbaden, 1871, Bd. i. S. 198 ; Paschutiu, ihkl, S. 304 ; Ewald, Virchoid's Arcliiv, 1879,
Bd. Ixxv. S. 409 ; Garland and Masloff, Uiitersitch. a. d. ijhysiol. Inst. d. Univ. Heidelberg,
1878, Bd. ii. S. 290 ; Brown and Heron, loc. cit. ; Dana, Med. Neios, Phila., 1882, vol. xli.
p. 59. ; Hamburger, Arch. f. d. ges. Fhijsiol., Bonn, 1S95, Bd. Ix. S. 560 ; Mendel, ibid.,
1896, Bd. Ixiii. S. 425. On the other hand, its existence is denied by Thiry and by Leube,
Jahresb. ii. d. Fortschr. d. ges. Med., Erlangen, 1868, Bd. i. S. 97. It must be remembered
that the diastatic action is admittedly a slight one by most of those observers wlio confirm
it, and that most organs and tissues possess a slight diastatic action, so that it is difficult to
be certain that the intestinal mucous membrane specifically secretes a diastatic ferment.

2 It is certain, from purely chemical experiments, that the acid is capable of producing such
an effect, and no inverting enzyme has ever been shown to exist in the gastric secretion.

3"Lehrbuch der physiol. Chem.," Aufl. 2. Bd. iii. S. 255; v. Becker, Ztschr. f.
wissensch. Zoologie, Bd. v. S. 123 ; J. Seegen, Arch. f. d. ges. Fhysiol., Bonn, 1887, Bd. xl.

S. 41.

•» Paschutin, Arch. f. Anat. u. Physiol, Leipzig, 1871, S. 306.

s Claude Bernard. "Le9ons sur la diabete et la glycogenese animale," Paris, 1877,
257-261.

6 Untprsuch. z. Naturl. d. Mensch. u. d. Thiere, 1889, Bd. xiii. S. 62 ; see also
Bastianelli, ibid., 1886, Bd. xiv. S. 146.

■^ "Handbuch der. jjath. u. physiol. chem. Analyse," 1893, Aufi. 6, S. 298. See also
Pautz and Vogel, Ztschr. f. Biol., Mtinchen, 1895, Bd. xxxii. S. 304.



DIGESTION OF PROTEIDS. 399

supposition is negatived by the rapidity of the action, and its progress
in presence of antiseptics ; and both by the recent observations of
Miura/ which show that the mucous memljrane of the intestine of
newly-born animals, under antiseptic conditions, causes inversion. No
inversion was obtained with the mucous membrane of the stomach or
large intestine.

Brown and Heron ^ have shown that the dried mucous membrane
of the small intestine is much more powerful, both in its diastatic action
on starch and in its inverting action on cane-sugar and maltose, than
are infusions of the same material. Starch also disappears from an
intestinal fistula (Thiry) much more rapidly than it is j)0ssible for the
succus eutericus,^ judging from other experiments, to convert it into
sugar. These facts point to a possibility that the epithelial cells of
the intestinal mucous membrane may possess the power of absorbing
starches and saccharoses, and submitting them to diastatic and inverting



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