EARLY BIOCHEMICAL EVENTS FOLLOWING 

 FERTILIZATION OF SEA URCHIN EGGSi 



David EpeP 



Johnson Research Foundation, Department of Biophysics and 



Physical Biochemistry, University of Pennsylvania Medical School, 



Philadelphia, Pennsylvania 



INTRODUCTION 



Fertilization results in a metabolic activa- 

 tion, similar in certain respects to the activa- 

 tions occurring upon neurochemical stimulation 

 of muscle or addition of hormone to target 

 tissue. It differs from the above, however, in 

 that fertilization occurs only once during the 

 lifetime of the organism, initiating a unique 

 series of reactions leading to rapid cell divi- 

 sions and embryonic differentiation. 



The changes which occur upon fertilization 

 are dramatic at both the morphological and 

 molecular levels. Changes in membrane struc- 

 ture, respiration rate, and rates of DNA, RNA, 

 and protein synthesis occur, as well as changes 

 in cation and coenzyme content, and subcellular 

 location of enzymes. These all occur within 

 seconds or minutes of insemination, and some- 

 how are interrelated with each other to yield an 

 orderly pattern of embryonic development. 



Although many post-fertilization changes 

 have been observed, numerous unresolved prob- 

 lems still exist. Little is known about how 

 these changes occur, when they occur, or the 

 casual connections between them. For example, 

 it is not known whether synchronous activation 

 of all enzymes is the case, or whether one or 

 several changes are triggered which then initiate 

 the other reactions in a chain or cascade-type 

 reaction system. 



The research to be discussed represents 

 the beginnings of an intensive study of the 

 fertilization reactions, aimed at shedding some 

 light on the above problems. The experimental 

 approach used is based on the assumption that 

 the fertilization changes result solely from 

 enzymic activation. The pertinent evidence for 

 this is, first, that eggs can be artificially ac- 



tivated (artificial parthenogenesis) to develop 

 without sperm (1). This indicates that the 

 sperm does not supply some missing enzyme or 

 substrate to the egg, and hence implies that all 

 materials necessary for development reside in 

 the egg. The second piece of evidence is that 

 eggs can be fertilized in the presence of con- 

 centrations of puromycin sufficient to inhibit 

 the bulk of protein synthesis. Under such con- 

 ditions, they will develop up to the first mitotic 

 division (90 minutes after insemination in the 

 eggs of S. purpuratus) before any arrest occurs 

 (2). This result means that little or no de novo 

 protein synthesis is required for the earliest 

 reactions of development, such as pronuclear 

 fusion or RNA synthesis. These two experiments 

 indicate that the immediate changes of fertiliza- 

 tion most probably result from activity of 

 enzymes already present in the egg. 



Enzymes and metabolic pathways activated 

 by fertilization, as well as physicochemical 

 changes possibly controlling these activations, 

 are shown in Table I. This table categorizes 

 the best described post-fertilization changes in 

 sea urchin eggs as changes in carbohydrate 

 and energy metabolism, co-factor and coenzyme 

 metabolism, synthetic metabolism, and changes 

 in structure. 



Examination of these changes suggests some 

 possible factors limiting metabolism in the 

 unfertilized egg. For example, the metabolic 

 machinery of the egg might be limited by cations 

 (as evidenced by changes in Ca"*"^ or K+), by 



^Supported by Public Health Service grant 5T1 GM2G277 

 and National Science Foundation grant GB-4206. 



^ Present address: Hopkins Marine Station, Pacific 

 Grove, California. 



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