Enzyme activities, during embryonic

The tissue distribution of adenine phosphoribosyltransferase varies considerably in rat, mouse, man, and monkey, and representative data are shown in Table 8-1. Murray 30) has found that the kinetic parameters of this enzyme in mouse liver vary during the course of embryonic development and has suggested that two forms of the enzyme may appear in succession. Both Murray 30) and Epstein 31) have observed marked changes in adenine phosphoribosyltransferase activity during embryonic development. 

The level of activity of enzyme synthesis and, consequently, the level of activity of the corresponding genes are mainly responsible for determining the changes in enzyme systems during embryonic development which we have just examined. 

Enzymes with an initially low activity which gradually increased during embryonic development. To this group of enzymes belong fructose-1,6-diphosphate aldolase (FDP-aldolase) and NAD-dependent malate dehydrogenase (NAD-MDH). The activities of these enzymes increased gradually until the morula-blastocyst transition, whereupon the enzyme activity increased more rapidly (Epstein et al., 1969). 

Another typical example of individual behavior of enzymes during embryonic development is shown in Fig. 78, demonstrating the dynamics of activity of four enzymes in the first week of development of the chick embryo (Moog, 1965). 

Later work (Nemeth, 1962 Nemeth and Haba, 1962) showed that both the embryonic and the inductive formation of tryptophan pyrrolase is dependent on systems of synthesis of proteins and RNA (i.e., is not simple activation of the proenzyme enzyme type) and is inhibited by inhibitors of synthesis of RNA and proteins. Neither the qualitative characters nor the homogeneity of the enzyme changed during ontogenesis and induction (Nemeth, 1961). 

Telomerase is an enzyme in eukaryotes used to maintain the telomeres. It contains a short RNA template complementary to the DNA telomere sequence, as well as telomerase reverse transcriptase activity (hTRT). Telomerase is thus able to replace telomere sequences that would otherwise be lost during replication. Normally telomerase activity is present only in embryonic cells, germ (reproductive) cells, and stem cells, but not in somatic cells. 

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