ABSTRACT
Cephalopod tissues contain a hitherto undescribed phosphagen compound.
The rate of acid hydrolysis is comparable with those of creatine and arginine phosphates, but is minimal in N/10 acid, while the retardation produced by the molybdate ion is only about 3 − 4 times.
The finding of a new compound in the Cephalopoda, the Lamellibranchiata being already known to contain arginine phosphate, is possibly of evolutionary significance.
INTRODUCTION
The comparative researches of Eggleton & Eggleton(5) on the distribution of phosphagen made it clear that while creatine phosphate is very widely distributed amongst the vertebrates, it is not present in the invertebrates. Shortly afterwards, a new phosphagenic substance was isolated from crab muscle by Meyerhof & Lohmann (11, 12) and shown to be arginine phosphate, while the later work of Lundsgaard (9) has made it certain that this compound plays in these tissues a part exactly analogous to that played by the creatine compound in vertebrate muscles. Later, Meyerhof (10) examined a number of invertebrates, representative of several phyla, and came to the conclusion that arginine phosphate is present in Holothuria, Pecten and Sipunculus. Cephalopod muscle contained no phosphagen.
During the work of Needham, Needham, Baldwin & Yudkin, one slight peculiarity was noticed in the behaviour of the phosphagen, and the authors’ attention was redirected to this on reading the paper by Iseki to which reference has already been made, and further investigations were carried out during a visit to the Marine Biological Station of Tamaris, Var, France.
The cephalopod chosen for most of the experiments was Eledone moschata, which closely resembles Octopus except that it possesses one row of suckers on the tentacles instead of two. Experiments on Sepia were also projected, but these animals seldom survived for more than 24 hours after being caught, and would therefore not have been in very good condition for experimental work, since it has been shown that their condition has a very marked effect upon the phosphagen content of the muscles (13). Eledone, on the other hand, lived much longer, and specimens used after 4 or 5 days in the aquarium seemed still to be in excellent condition.
When required for an experiment an animal was taken and placed in a large bowl filled with ice and left thus in the ice-chest for 20 min. or longer, according to the size of the animal. A suitable piece of muscle was cut away from the mantle, the animal having become perfectly quiet, then freed from skin, and dropped into an ice-cooled beaker, weighed, and extracted with ice-cold 10 per cent, trichloracetic acid by grinding as rapidly and thoroughly as possible with acid-washed quartz sand. The extracts were filtered under pressure through cooled Gooch crucibles of 16 c.c. capacity, and received in a cooled tube, and used at once for the experiments. The method of Fiske & Subbarow(6) was used throughout for the estimations of the phosphate, and a Klett top-reading colorimeter was employed for the comparisons.
EXPERIMENTS
Exp. 1. 4·51 gm. of tissue were extracted with 20 c.c. of the trichloracetic acid. The filtered extract was kept at 0° C. and the pH adjusted by adding saturated soda till the colour of a little added thymol blue showed some visible change, and then determining it more accurately by means of a capillator. The value found was 1-7. The whole extract was now rapidly heated to 28° C. by immersion in a warm bath, and then transferred to an incubator at the same temperature. The variation observed in the temperature of the incubator, which was electrically regulated, was ± 0·5° C. during the experiment. 1 c.c. samples were thereafter removed and the phosphorus content determined. In addition, two samples were put up with onetenth of their volume of 2·5 per cent, ammonium molybdate, and these were also incubated, and their phosphate contents determined in order to get some idea of the effect of the molybdate ion upon the reaction velocity.
The results of this experiment are given in graphical form in Fig. 1, and show that the hydrolysis is complete in about 12 hours under the conditions of the experiment. The velocity constant for the hydrolysis (calculated in terms of natural logarithms with the minute as time unit) is 6·2 × 10−3 for 50 per cent, hydrolysis, a value which agrees well with those in the literature for arginine phosphate. The lower curve in Fig. 1 connects the points obtained in the presence of the molybdate ion, allowance having been made for the dilution of the samples by the addition of the molybdate, and shows that the only effect of the ion is a slight retardation.
In addition, a rough estimate of the total free guanidine base of the extract was obtained by the method of Weber (18) which is based on the Sakaguchi reaction (17).
The base present in the solution gave a coloration which showed no visible difference in tint from that produced in a standard solution of arginine, and it was supposed that the intensity of the colour produced by equivalent amounts of arginine and of the free base would be the same. On this assumption it was possible to calculate the amount of phosphate to which any given amount of base would be equivalent, and thus, in the later experiments, to compare the amounts of base and phosphate liberated in any given time in the same solution. In the present case, the amount of free base corresponded to 0·294 mg. P Per c.c. solution, while the total free phosphate was 0·263 mg. per c.c., indicating the presence of a slight excess of free base. The same is the case in frog muscle according to Dulière (4), and in the electrical organ of Torpedo according to the present author (3).
Returning to the question of the velocity constants ; it has been mentioned that the molybdate ion produced only a small retardation of the hydrolysis. According to Meyerhof & Lohmann (12) the hydrolysis of arginine phosphate is retarded some 30 times by this ion, or rather less in crude extracts, which suggests that the phos-phagen is not arginine phosphate, as was previously thought. But on the other hand, the molybdate ion is known to have a marked catalytic effect upon a large number of phosphoric compounds, and it was possible that some such compound was breaking down under the influence of the molybdate and masking the effect of the latter upon what might really have been the arginine compound. This possibility was investigated in the next experiment.
Exp. 2. An extract was prepared from 4·82 gm. of muscle with 25 c.c. of trichloracetic acid. 2·5 c.c. of 2·5 per cent, ammonium molybdate were added to the extract after filtration, when a slight turbidity resulted which, however, cleared up in the subsequent operations. The pH was brought to 1·6 and the solution heated to 28° C. as before and transferred to the incubator. The fluid was now perfectly clear, i c.c. samples were taken at intervals for the estimation of the free base, and further i c.c. samples for the estimation of the phosphate. (It has been found by the author and Dr D. M. Needham that the Sakaguchi reaction is given by free arginine, but not by arginine combined in the form of the phosphate.) The results obtained are given in Fig. 2. The estimations of the base are much less accurate than those of the phosphate, for the method used in the former case is accurate at best to only 3 − 5 per cent. But none the less, it is evident that the amount of guanidine base liberated is approximately equivalent to the phosphate and that the amount of phosphate contributed by the breakdown of other phosphorus compounds is at any rate small.
It may therefore be taken that the velocity constant calculated from the phosphate curve, which was 1·9 × 10−3, refers to the hydrolysis of the phosphagen and not to that of another compound.
Exp. 3. The third experiment, the results of which are given in Fig. 3, was a repetition of the first, except that the base liberation as well as that of the phosphate was followed. 4·25 gm. of tissue were taken and extracted with 20 c.c. of the tri-chloracetic acid. The curves show that the base and the phosphate are here again liberated in equivalent amounts, thus disposing of the possibility that two compounds are present which are differently affected by the molybdate ion. The k value calculated from the phosphate curve is 7·1·10−3, the temperature being 28° C. and the pH 1·5 ; the average value of k from this and the first experiment is therefore 6·6·10−3. In the presence of the molybdate ion the value was 1·9·10−3, indicating that the ion in question produces a retardation of about times.
Finally, the results of the three experiments are given together in Table I, together with some data for other Cephalopoda taken from Needham, Needham, Baldwin & Yudkin(13). The figures for the different P fractions represent the amounts of P in mg. per gm. of tissue, and are not corrected for the water content of the muscle.
Exp. 4. The effects of acidity upon the rates of hydrolysis were next investigated. 4.60 gm. of muscle were extracted with 20 c.c. of acid in the usual manner, and after filtration the extract was neutralised to phenol red. 1 c.c. samples were then taken and incubated at different acidities for 120 min. at 28° C., initial and final determinations of the phosphate content being also made. A series of values for the velocity constant of the hydrolysis were calculated and are given in Table II.
Exp. 5. 7.30 gm. of the mantle muscle of a specimen of Octopus were extracted with 20 c.c. of trichloracetic acid, the extract being filtered and neutralised to phenol red, etc., as in the last experiment. The velocity constants are given in Table II.
It will be seen that in both cases the constant has a minimal value in N/10 solution. Meyerhof & Lohmann (12) have shown that in the case of pure solutions of arginine phosphate the constant has a maximal value in N/100 solution, or, according to Meyerhof (10), at N/10 in crude extracts. For purposes of comparison, the data of Table II have been plotted in Fig. 4, together with Meyerhof’s data (10) for some other invertebrates.
DISCUSSION
The phosphagen of Eledone differs from arginine phosphate in two important respects. In the first place, its hydrolysis is much less profoundly affected by the molybdate ion than is that of the arginine compound ; in both cases a retardation is produced, of about 30 times in the case of arginine phosphate, and of about 3 J times in the case of the phosphagen of Eledone. In the second place, acidity alters the rates of hydrolysis in both cases, but whereas the hydrolysis rate is minimal in N/10 acid solution in the case of the compound of Eledone, it is maximal at the same acidity in the case of arginine phosphate.
The latter was found to be the case in Octopus as well as in Eledone, and this raises the question as to how far it can be supposed that the phosphagen compound here present is to be found amongst the Cephalopoda. It seems that at any rate we may suppose it to be present in Sepia and Octopus as well as in Eledone, for if we do so it at once explains the anomaly observed by Needham, Needham, Baldwin & Yudkin (13) in the behaviour of the phosphagen of these forms. In the estimations of the “creatine fraction’’ of the phosphate of the extracts, a solution containing all the phosphagen of the solution is allowed to stand for about 15 − 20 min. in the presence of a strongly acid solution of ammonium molybdate, inorganic phosphates being previously removed by precipitation by means of calcium at pH 9. If creatine phosphate is present, it is broken down completely in 15 min., whereas, if arginine phosphate is present, the phosphagen does not break down appreciably on account of the fact that the molybdate exerts an inhibitory effect upon its hydrolysis. But in the case of the cephalopod phosphagen, the inhibition produced is only one-tenth of that in the case of the arginine compound, and consequently, enough of it could be broken down in 15 min. to give a detectable amount of phosphate in the so-called creatine fraction, and the possibility would of course be greater when the amount of phosphagen present in the fraction was great to start with. In such cases as this phosphate appeared in the creatine fraction. This explanation of the apparent anomaly is certainly better than the supposition that all the inorganic phosphate had not been precipitated, for the amount of calcium added was certainly in considerable excess of the amount of phosphate to be precipitated.
Evidently then, we may suppose that the same compound is present in all the cases so far examined, and it is probable that the same compound is to be expected in other Cephalopoda. The supposition that it really is a new compound is certainly favoured by Iseki’s finding of a new compound in place of the arginine commonly found in the invertebrates, and an isolation of the phosphagen will be attempted as soon as possible.
With regard to the other mollusca, not much can be said at the present moment. Meyerhof (10) has examined a number of molluscs, and has been unable to detect any phosphagen in Sepia, or in Octopus. Amongst the lamellibranchs he could find none in Pinna, Cytherea or Anodonta, but in Pecten he has shown that arginine phosphate is present, while Ackermann (1) claims to have isolated arginine from Mytilus edulis. What results are to be expected of the Gastropoda it is at present impossible to say, for with the exception of Aplysia, which Eggleton & Eggleton (5) found to contain no creatine phosphate, none seems to have been examined.
To find such a difference between the members of the two groups, viz. the Cephalopoda and the Lamellibranchiata, is interesting in view of the theory of the evolution of the groups in question, for it is believed that the Lamellibranchiata left the main line of evolution at a much earlier date than the other two groups, and it will be interesting to see how the Gastropoda fit into the scheme. The mollusca as a whole are well known to be unusual in their muscle chemistry—see for example Ritchie (16)—and it is evident that a careful survey of the muscle chemistry from the comparative point of view must lead to results of great interest. Fortunately, a wide range of molluscs is readily available throughout the country and it is hoped that studies upon their musculature will be carried out in the near future.
ACKNOWLEDGMENTS
The author’s grateful thanks are due to Prof. H. Cardot, the Director of the Tamaris Station, for his generous welcome and great kindness, and to Prof. Jean Roche and Madame Roche for their unfailing interest, which greatly aided the work. Financial assistance, granted from the Worts Fund of the University of Cambridge, and from the United Services Fund, is also gratefully acknowledged.