vol. iii, p. 37.
_Proteolysis of Gelatin._[133]
==+===============================================
|Mother Proteid.|Protogelatose.|Deuterogelatose.
--+---------------+--------------+----------------
C | 49.38 | 49.98 | 49.23
H | 6.81 | 6.78 | 6.84
N | 17.97 | 17.86 | 17.40
S | 0.71 | 0.52 | 0.51
O | 25.13 | 24.86 | 26.02
--+---------------+--------------+----------------
[133] Chittenden and Solley: Journal of Physiol., vol. xii, p. 33.
_Proteolysis of Phytovitellin_[134] _(Crystallized) from Squash Seed._
==+===============+===============+=================
|Mother Proteid.|Protovitellose.|Deuterovitellose.
--+---------------+---------------+-----------------
C | 51.60 | 51.52 | 49.27
H | 6.97 | 6.98 | 6.70
N | 18.80 | 18.67 | 18.78
S | 1.01} | |
O | 21.62} | 22.83 | 25.25
--+---------------+---------------+-----------------
[134] Chittenden and Hartwell: _Ibid._, vol. xi, p. 441.
_Proteolysis of Phytovitellin_[135] _(Crystallized) from Hemp Seed._
=============================================================
|Mother Proteid.|Protovitellose.|Deuterovitellose.|Peptone.
--+---------------+---------------+-----------------+--------
C | 51.63 | 51.55 | 49.78 | 49.40
H | 6.90 | 6.73 | 6.73 | 6.77
N | 18.78 | 18.90 | 17.97 | 18.40
S | 0.90 | 1.09 | 1.08 | 0.49
O | 21.79 | 21.73 | 24.44 | 24.94
--+---------------+---------------+-----------------+--------
[135] Chittenden and Mendel: _Ibid._, vol. xvii, p. 48.
_Proteolysis of Glutenin_[136] _from Wheat._
==+=========================================
| Mother | Proto- | Hetero- | Deutero-
|Proteid.|glutenose.|glutenose.|glutenose.
--+--------+----------+----------+----------
C | 52.34 | 51.42 | 51.82 | 49.85
H | 6.83 | 6.70 | 6.79 | 6.69
N | 17.49 | 17.56 | 17.43 | 17.57
S | 1.08 | 1.34 | 1.59 | 0.80
O | 22.26 | 22.98 | 22.37 | 25.09
--+--------+----------+----------+----------
[136] Formerly called gluten-casein, and the products gluten-caseoses.
Chittenden and E. E. Smith: Journal of Physiol., vol. xi, p. 420.
_Proteolysis of Zein._[137]
============================================
|Mother Proteid.|Protozeose.|Deuterozeose.
--+---------------+-----------+-------------
C | 55.23 | 53.29 | 51.31
H | 7.26 | 6.87 | 6.88
N | 16.13 | 16.10 | 16.27
S | 0.60 | 1.54 | 1.08
O | 20.78 | 22.20 | 24.46
--+---------------+-----------+-------------
[137] Chittenden and Williams: Not heretofore published.
In considering these results, it is to be noticed that there is a
general unanimity of agreement except in the case of the albuminoid
gelatin. In the proteolysis of this body, for some reason not
explainable, the digestive products show no marked deviation from the
composition of the mother-proteid, but in every other instance there
is to be traced a distinct tendency toward diminution in the content
of carbon, proportional to the extent of proteolysis. In the primary
bodies, proto and heteroproteoses, the percentage of carbon is only
slightly lowered; indeed, in some few cases, notably in elastin and
casein, the primary products show a slight increase in their content
of carbon, but in most instances there is a slight falling off in the
percentage of this element. In the deuteroproteoses, however, the loss
of carbon is very marked. The percentage loss, to be sure, varies with
the different proteids, doubtless dependent in part upon the nature
of the proteid itself, and also, I think, upon the strength of the
proteolytic agent employed and the duration of the proteolysis. It is
to be further noticed that peptones, whenever analyzed, show a still
further loss of carbon and also a marked loss of sulphur. In nitrogen
there is no constant difference.
On the assumption that these various products of proteolysis are
formed by a series of hydrolytic changes, accompanied by cleavage of
the molecule, we might at first glance look for a marked increase in
the content of hydrogen. But when we consider the size of the proteid
molecule, with the small proportion of hydrogen contained therein
and the large amount of carbon, it is plain that hydrolytic cleavage
might naturally leave its mark on the percentage of carbon, rather
than on the percentage of hydrogen of the resultant products. In view
of these facts, the above results show nothing inconsistent with the
theory that pepsin-proteolysis, as a rule, is accompanied by a series
of progressive hydrolytic cleavages in which the primary proteoses are
the result of a slight hydration, these bodies by continued proteolysis
being further hydrated with formation of secondary proteoses, which in
turn undergo final hydration and cleavage into true peptones. In accord
with this theory, true peptones always show a marked difference in
composition from that of the mother-proteid, the most striking feature
being the greatly diminished content of carbon, which may be taken as
a measure, in part at least, of the extent of the hydrolytic change.
And it is to be noticed that the crystallized phytovitellins are no
exception to the general rule; the secondary vitelloses and peptones
resulting from proteolysis bear essentially the same relationship to
the mother-proteids that the albumoses from egg-albumin do. Moreover,
the alcohol-soluble proteids, of which the zein of cornmeal is a good
example, show the same general tendency, and it is an interesting
fact that the proteoses, or more specifically the zeoses, formed from
this peculiar proteid, are readily soluble in water and show the
general proteose reactions. It may also be mentioned that these zeoses,
as well as the elastoses, are very resistant to further hydrolysis
by pepsin-acid, and yield only comparatively small amounts of true
peptones.
In connection with this question of the composition of proteoses
and peptones as formed by pepsin-proteolysis, it is interesting to
note a recent observation recorded by Schützenberger.[138] This
experimenter took 350 grammes of moist blood-fibrin, corresponding to
75.5 grammes of dry substance, and subjected it to proteolysis with
2.5 litres of a very strong pepsin-hydrochloric acid solution for five
days. The resultant fluid was then freed from acid by treatment with
silver oxide, after which the solution was evaporated to dryness on
a water-bath and the residue dried _in vacuo_. This residue, termed
by Schützenberger fibrin-peptone, was found on analysis to contain
49.18 per cent. of carbon, 7.09 per cent. of hydrogen, and 16.33 per
cent. of nitrogen, thus agreeing very closely with true fibrin-peptone
as analyzed by Kühne and myself. Further, Schützenberger showed that
the fibrin in undergoing this transformation had taken on 3.97 per
cent. of water. But to my mind, the most significant fact connected
with this experiment is the positive evidence it affords, not only of
hydration as a feature of peptonization by pepsin-acid, but that this
greatly diminished content of carbon, so characteristic of peptones,
and to a less extent of deuteroproteoses, is wholly independent of the
methods of separation and purification ordinarily made use of. Thus,
Schützenberger, in the above experiment, did not attempt any separation
of individual bodies. Proteolysis was carried out under conditions
favoring maximum conversion into peptone, and the resultant product,
or products, was analyzed directly without recourse to any methods
of precipitation or purification. To be sure, the substance analyzed
could not have been peptone entirely free from proteose, but in any
event it represented the terminal products of pepsin-proteolysis, and
like true amphopeptone contained 3.5 per cent. less carbon than the
original fibrin. Hence, we may conclude, without further argument, that
peptonization in gastric digestion is the result of distinct hydrolytic
action, in which the original proteid molecule is gradually broken
down, or split apart, into a number of simpler molecules, the proteoses
and peptones.
[138] Recherches sur la constitution chimique des peptones. Comptes
Rendus, vol. 115, p. 208.
Peptones, _i. e._, amphopeptones, are the final products of gastric
digestion; but to how great an extent is actual peptonization carried
on in pepsin-proteolysis? As we have seen, syntonin, primary proteoses,
secondary proteoses, and peptones are all products of pepsin-digestion,
and it might perhaps be assumed that ultimately all of a given proteid
undergoing pepsin-proteolysis would be converted into amphopeptone.
Examination, however, shows that such is not the case, at least in
artificial digestive experiments. Peptones are truly formed, and
many times in large amount, but never under any circumstances have
I been able to effect a complete transformation of any proteid into
true peptone by pepsin-proteolysis; there is always found a certain
amount of proteoses more or less resistant to the further action of
the ferment. Obviously, the nature and proportion of the individual
products formed in any digestive experiment are dependent greatly
upon the attendant conditions; but even with a large amount of active
ferment, an abundance of free hydrochloric acid, a proper temperature,
and a long-continued period of digestion, even five and six days,
there is never found a complete conversion into peptone. Indeed,
the largest yield of peptone I have ever obtained in an artificial
digestion is sixty per cent., while the average of a large number of
results under most favorable circumstances is somewhat less than fifty
per cent.[139]
[139] Chittenden and Hartwell: The Relative Formation of Proteoses and
Peptones in Gastric Digestion. Journal of Physiol., vol. xii, p. 12.
We understand that peptones are the products of the hydration and
cleavage of previously formed proteoses. The primary proteoses pass
into secondary proteoses and these into peptones, but for some reason
this transformation after a time becomes a slow and gradual process. At
first there is a marked and rapid progression; the proteid undergoing
proteolysis is rapidly dissolved, and both proteoses and peptones may
be detected in abundance. But if we continue to watch the changing
relations of primary and secondary proteoses and peptones, we find
that progression soon ceases to be rapid, and eventually travels
onward at a snail’s pace. Thus, in one experiment with coagulated
egg-albumin, there was found at the end of forty-eight hours’ digestion
with pepsin-hydrochloric acid, only thirty-seven per cent. of peptones
with fifty-eight per cent. of proteoses, and yet digestion had been
sufficiently vigorous to allow of a complete solution of the proteid
in two hours. At the end of seventy-two hours the amount of peptones
had increased to about forty-two per cent., the proteoses having
correspondingly diminished; but even at the end of seventeen days only
fifty-four per cent. of peptones were to be found, thus affording
striking evidence of the slow conversion of the first-formed products
into peptones.
Naturally, the individual proteoses show marked differences in their
rate of conversion into secondary or final products. Take as an
illustration some results[140] obtained with caseoses formed in the
digestion of the casein of milk. Thus, heterocaseose, a primary
product, yielded only fifteen per cent. of peptone after ninety-four
hours at 40° C. with a strong pepsin-acid solution. Protocaseose,
however, containing some deuterocaseose, under like conditions, yielded
thirty-two per cent. of peptone in one hundred and nineteen hours,
while pure deuterocaseose gave sixty-six per cent. of peptone in one
hundred and thirty-seven hours. Evidently, then, the first-formed
soluble products of gastric digestion, _i. e._, the primary proteoses,
are only slowly converted into peptone, since they must first pass
through the intermediate stage of deuteroproteose, which is plainly
not a rapid process. The deutero-body, on the other hand, once formed
is more rapidly converted into peptone, but even this is in no sense
a rapid process. Hence, in the artificial digestion of proteids with
pepsin-hydrochloric acid, solubility of the proteids may be quite
rapid, and even complete in a very short time, but the resultant
products will be mainly proteoses and not peptones. The latter are
truly formed and in considerable amount, but proteoses, either as
primary or secondary bodies, are invariably present and usually in
excess of the peptones.
[140] Chittenden and Hartwell, loc. cit., p. 22.
In this connection the question naturally arises how far we are
to trust these results in their bearing on the natural process of
digestion as it occurs in the living stomach. Obviously, the conditions
are quite different in the two cases. In artificial digestions, we
have especially the influence of an ever-increasing percentage of
soluble products on the activity of the ferment, a condition of things
generally considered as more or less inhibitory to enzyme action.
We have attempted to measure the real value of this influence by
experiments[141] conducted in parchment dialyzing tubes, in which
the conditions are made favorable for the removal of at least some
of the products of digestion as fast as they are formed. In these
experiments, the dialyzer tubes containing the proteid and pepsin-acid
were immersed in a large volume of 0.2 per cent. hydrochloric acid
(about three litres), which was gradually changed from time to time,
the whole mixture being kept at 40° C. during the entire period of
the experiment. The extent of peptonization was then ascertained
by analysis of both the contents of the dialyzer tubes and of the
surrounding acid, the results being compared with those obtained from
control experiments carried on in flasks. Without considering the
results in detail, it may be mentioned that the slow and incomplete
peptonization so characteristic of artificial gastric digestion is not
materially modified by this closer approach to the natural process.
The several digestions carried on in the dialyzer tubes were certainly
accompanied by a fairly rapid withdrawal of the diffusible products of
digestion, yet no noticeable increase in the amount of peptone formed
was observed. The results certainly favor the view that the conversion
of the primary products of gastric digestion into true peptone is a
slow and gradual process, even under the most favorable circumstances,
and that this lack of complete peptonization is not due to accumulation
of the products of digestion, but is rather an inherent quality of
pepsin-proteolysis under all circumstances.
[141] Chittenden and Amerman: A Comparison of Artificial and Natural
Gastric Digestion, together with a Study of the Diffusibility of
Proteoses and Peptones. Journal of Physiol., vol. xiv, p. 483.
In these dialyzer experiments it was observed that not only did
peptones diffuse, but also the proteoses. In fact, it was found that
six to eight per cent. of the proteoses formed passed through the
parchment walls of the dialyzer tubes into the surrounding acid in
the nine hours’ digestion. This led to a study of the diffusibility
of proteoses in general, from which we were led to conclude that
these bodies possess this power to a greater degree than had hitherto
been supposed. As might be expected, it was also found that the
attendant conditions modify materially the rate of diffusibility; the
two factors especially prominent being temperature and the volume of
the surrounding fluid. Thus, 1.9 grammes of protoalbumose dissolved
in 200 c.c. of water and suspended in 4.5 litres of water heated to
38° C., diffused through the parchment tube to the-extent of 5.09 per
cent., while at 10° C. diffusion amounted to only 2.57 per cent. Under
somewhat similar conditions, pure peptone diffused to the extent of
eleven per cent. in six hours at 38° C. Somewhat singular, however,
was the result obtained with deuteroalbumose; this proteose showing a
diffusibility considerably less than that of the proto-body. But as
Kühne[142] has independently obtained essentially the same results,
this apparent anomaly cannot depend upon any errors of work.
[142] Erfahrungen über Albumosen und Peptone. Zeitschr. f. Biol., Band
29, p. 20.
It is of course to be understood that diffusion experiments made with
dead parchment membranes cannot necessarily be expected to throw much
light upon the rate of absorption of these bodies through the living
membranes of the stomach and intestine, where, as Waymouth Reid[143]
has well said, we have to deal with an absorptive force dependent, no
doubt, upon protoplasmic activity, and comparable, in part at least,
to the excretive force of a gland-cell. Furthermore, in considering
absorption as it occurs in the living stomach, we must necessarily
give due weight to the selective power of the epithelial cells,
a power which may be far more potent even than we suppose. Hence,
without attempting at this point to draw any broad deductions from our
experiments we may simply lay stress upon the facts themselves, viz.,
that the primary products of pepsin-proteolysis are diffusible, and,
like true peptones, are capable of passing through animal and vegetable
membranes, although to a less extent. We may further emphasize the
fact that experiments of this character on diffusibility can, at the
most, only indicate general tendencies, since every variation in the
attendant conditions will exercise some influence upon the final result.
[143] Osmosis Experiments with Living and Dead Membranes. Journal of
Physiol., vol. xi, p. 312.
With reference to the bearing digestive experiments made in dialyzer
tubes have upon the natural process as carried on in the living
stomach, we must necessarily grant that the conditions approximate
only in the crudest way to those existent in the alimentary tract.
At the same time, if complete peptonization is characteristic of
pepsin-proteolysis in the stomach, and failure to obtain such results
in an artificial digestion is due to lack of withdrawal of the
diffusible products formed, then certainly the experiments carried on
in dialyzer tubes, with abundant opportunity for diffusion, and with a
large excess of free hydrochloric acid, should show some indications of
increased peptone-formation. But none were obtained.
It is more than probable that the rate of absorption of diffusible
products from the stomach has been overestimated. Lea,[144] for
example, assumes that, “normally the products of digestion, whether
proteid or carbohydrate, are never met with in either the stomach
or intestine in other than the smallest amounts, frequently to be
described as merely traces.” This certainly implies a far more rapid
absorption of proteoses and peptones from the stomach than results
seem to justify. Indeed, recent facts obtained by Brandl,[145] working
under Tappeiner’s direction, tend to show that absorption from the
stomach is, under some circumstances at least, comparatively slow.
Brandl’s experiments were conducted on large and vigorous dogs with
gastric fistulæ, the stomach being shut off from the intestine by the
simple introduction of a small rubber balloon into the pylorus, which
when dilated completely closed the orifice. By carefully conducted
experiments, it was shown that pure peptone, entirely free from
proteose, is absorbed from the empty stomach in proportion to the
concentration of the peptone solution. Thus, 7.5 grammes of peptone
dissolved in water in such proportion as to make a five per cent.
solution, and allowed to remain in the stomach for two hours, lost by
absorption only 0.28 gramme, equal to 2.68 per cent. of the peptone
introduced. Under similar conditions, a ten per cent. aqueous solution
of peptone lost only 4.5 per cent. by absorption. On the other hand,
when peptone was introduced in larger quantity, viz., in a twenty per
cent. solution, absorption amounted to thirteen per cent. in two hours.
[144] Journal of Physiol., vol. xi, p. 240.
[145] Ueber Resorption und Secretion im Magen, und deren Beeinflussung
durch Arzneimittel. Zeitschr. f. Biol., Band 29, p. 277.
It is thus evident that pure peptones, even when taken into the stomach
in fairly large amounts, and under conditions very favorable for rapid
absorption, pass into the circulating blood very slowly. Obviously,
however, one must not lose sight of the fact that when digestion is
under way and the volume of blood consequently increased, there may
be a corresponding rise in the rate of absorption. There is perhaps a
hint of this conclusion in the influence of alcohol on the absorption
of peptone as brought out by some of Brandl’s experiments. Thus, it
was found that when alcohol was added in considerable quantity to a
ten per cent. solution of peptone, the stomach-mucosa was greatly
reddened, while in two hours the absorption of peptone amounted to
11.8 per cent. But in any event, these results certainly do not favor
the view that the products of gastric digestion are absorbed as soon
as they are formed. It is, no doubt, quite different in the intestine,
but in the stomach, where pepsin-proteolysis occurs, we have, I think,
no grounds for assuming that either peptones or proteoses are rapidly
absorbed. Hence, it might perhaps be considered that the results of
pepsin-proteolysis in the living stomach are much the same as those
obtained in artificial digestion experiments.