Cellular Metabolism 



87 



the low rate of oxygen uptake of frogs' eggs 

 can be raised merely by homogenizing 

 (Spiegelman and Steinbach, '45). 



The most thoroughly explored case of svid- 

 den activation in normal development is that 

 of the fertilization of sea urchin eggs. In this 

 field we owe most of our knowledge to the 

 laboratory of Professor Rvmnstrom, who has 

 recently reviewed the work (cf. Runnstrom, 

 '49). We have no room here to do more than 

 point out some of the major evidences that 

 the first few minutes following sperm en- 

 trance involve simultaneovis and no doubt 

 correlated changes in enzymatic activity and 

 structural characteristics. During this period 

 the rate of oxygen uptake is raised or at 

 least stabilized (Borei, '48), with the prev- 

 iously immobilized cytochrome system com- 

 ing into action; there is a rapid breakdown 

 of high molecular weight carbohydrates (Or- 

 strom and Lindberg, '40) ; and nonprotein ni- 

 trogen increases rapidly (Orstrom, '41). At 

 the same time calcium is liberated, appar- 

 ently from a proteinate (Mazia, '37); an 

 increasing amount of protein becomes insolu- 

 ble in molar potassium chloride (Mirsky, 

 '36) ; and cephalin changes to an ether-insol- 

 uble form (Ohman, '44). The egg surface is 

 strongly affected, the thin jelly-coated vitel- 

 line membrane giving way to the tough, 

 birefringent fertilization membrane. Of 

 course the practical ovitcome of all this post- 

 fertilization activity is the lifting of the block 

 to development and the initiation of cleav- 

 age. In this whole system, it would seem, we 

 have the most favorable material now avail- 

 able* for showing on the one hand the rela- 

 tionship between protein structure and en- 

 zyme activity, and on the other the nature of 

 the linkage between the structure-controlled 

 activity and the events of development. 



Sudden alterations of activity are, however, 

 probably a minor aspect of the life of the 

 embryo. The proper business of an embryo 

 is differentiation, and one would expect that 

 the enzymatic orientations of the embryo dif- 

 ferentiate in the same sense as does the body 

 structure itself. In a few cases we have evi- 

 dence that biochemical elaboration of formed 

 elements does occizr. 



* The much-studied diapause state of the orthop- 

 teran egg has previously seemed to be an equally- 

 useful case of inhibition of development paralleled 

 by immobilization of enzyme systems. The recent 

 finding that the respiratory intensity of diapause 

 homogenates is lower than that of homogenates 

 from active stages (Bodine, '50) indicates, however, 

 that the situation is more complex than previously 

 supposed. 



The granular structvue of embryonic cy- 

 toplasm is the one problem in this field that 

 has been studied with any degree of thor- 

 oughness. The real existence of granules in 

 intact embryonic cells has recently been 

 shown by experiments in which P^- was in- 

 jected at unspecified stages into hen's eggs 

 or gravid mice (Jeener, '49). In embryos 

 homogenized two hours after the injection, 

 the specific activity of radiophosphate was 

 found to be differentially distributed among 

 the nucleic acids of granules isolated at 

 13,000 g. and at 60,000 g., and of the fmal 

 supernate. Since the differences are too great 

 to be accounted for merely by the exchange 

 rate, it appears that the granules are not 

 merely artifacts accumulated during the 

 homogenization process, but occur as real 

 entities in the intact cells. 



In the chick embryo from seven to eleven 

 days of incubation, centrifugable granules 

 accumulate an increasingly large portion of 

 the total nitrogen, and at the same time be- 

 come relatively richer in apyrase and alka- 

 line phosphatase activity (Steinbach and 

 Moog, '45; Moog and Steinbach, '46). The 

 granules of amphibian eggs gradually bind 

 all of the nucleoprotein, which is entirely 

 free at the beginning of development, and 

 they also become associated with catalase, 

 dipeptidase, ribonuclease, and alkaline phos- 

 phatase, although they are, surprisingly, said 

 to be poor in respiratory enzymes (Brachet 

 and Chantrenne, '42); it is unfortunate that 

 these fascinating results have never been 

 well documented. In sea urchin eggs a num- 

 ber of enzymes are bound to granules, but 

 others are dispersed in the hyaloplasm (cf. 

 Holter, '49). It is not known, however, if 

 the enzymatic character of the granules 

 changes during development. 



Among the few other instances of biochem- 

 ical elaboration of structure during develop- 

 ment, the most interesting is the finding that 

 in the muscles of the rat, just before and 

 after birth, apyrase accumulates at a faster 

 rate than myosin (Herrmann and Nicholas, 

 '48). The enzyme activity, it appears, is added 

 to the protein structure as warranted by 

 the needs of the whole organism. A similar, 

 though less clear, case is the shift in cal- 

 cium activability of apyrase in homogenates 

 of chick liver. Under the same conditions of 

 preparation, the enzyme is slightly inhibited 

 by calcium before hatching, but strongly 

 stimulated after hatching (Moog, '47); as 

 pointed out earlier, a difference in structural 

 association could account for such an effect. 

 A recent study correlating quantitative and 



