Quantitative Studies on Enzyme Repression in the Developing Chick Embryo and Newly Hatched Chick.

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Abstract
Summary Modification of the physical state of injected creatine has permitted a quantitative study of repression of arginineglycine transamidinase by creatine, and its precursors, in liver of the developing chick embryo. Half-maximal repression was obtained by 65 μmoles/egg of creatine and 45 μmoles of guanidinoacetate. Complete repression occurred with 260 and 120 μmoles, respectively. Betaine and 4-aminobutyrate decrease the degree of repression attained. Arginine, and its precursor, citrulline, repress 50% at 290 μmoles/egg; when glycine is injected with either of these compounds, repression is increased to 80%. Ornithine, which cannot be converted to citrulline in the chick, does not repress. Laying hens fed creatine and guanidinoacetate have embryos with normal transamidinase levels. For the first time, essentially complete enzyme repression has been attained throughout embryonic development. However, derepression occurs during the 6 days following hatching, when chicks are fed a complete diet. In absence of food, or in presence of dietary creatine, this increase does not occur. The possible significance for theories of differentiation of the demonstration of enzyme repression during embryonic development is discussed.

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