DEGRADATION OF LACTATE DEHYDROGENASE ISOZYMES INJECTED INTO FROG EGGS

Lactate dehydrogenase (LDH) isozymes from beef tissues were injected into newly fertilized frog eggs. Embryos at various stages were examined by starch gel electrophoresis to determine whether the injected isozyme was present. Beef LDH‐1 was stable for at least twelve days following injection, while beef LDH‐5 was present in the embryos as a functional molecule for less than six days. These very different in vivo stabilities are not in agreement with the in vitro stabilities obtained here, and suggest that regulated differential stability of isozymes may be important to the organism in conferring greater metabolic flexibility during cell differentiation when the isozyme repertory commonly changes. During the degradation of beef LDH‐1, no hybrid beef‐frog LDH molecules could be detected although such molecules can be formed by a simple in vitro technique. Since frog LDH is synthesized during the degradation of the injected LDH, the lack of detectable hybrid molecules indicates that the subunits of the degraded molecules cannot be reused—either because the molecule is degraded as a whole, without the release of subunits, or because the sites of synthesis and degradation are mutually inaccessible. Copyright © 1969 Wiley‐Liss, Inc., A Wiley Company