The formation and stabilization of an adaptive enzyme in the absence of its substrate.

It is shown that various substrates accelerate the disappearance of an adaptive enzyme when its own substrate has been removed from the medium. The order of effectiveness of such substrates appears to be connected with their chemical similarity to the adaptive substrate. It is shown that two conditions which are able to inhibit the formation of adaptive enzymes-anaerobiosis and the presence of sodium azide-are equally able to prevent the disappearance of an adaptive enzyme after the removal of its substrate. Finally, it is shown that rapidly growing cultures, under optimal conditions for synthetic activity, are able to maintain and even appreciably to increase their initial content of an adaptive enzyme, in the absence of its specific substrate and in the presence of a normally competitive substrate. In the light of these results, the three major theories of enzyme formation hitherto proposed are evaluated.