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A recent report from this laboratory ( Segal, Abraham,and Schatz, 1968 ) described some in vitro stability studies with rat liver alanine aminotransferase in which it was found that L-proline protected the enzyme against heat denaturation better than any other amino acid tested. It was of interest to us to pursue this observation in order to decide whether the interaction was at the active site, and if so whether a complex was formed with the pyridoxal moiety. The results allow us to conclude that L-proline does interact with the prosthetic group and to speculate about the nature of the complex and its relationship to normal intermediates in the transamination reaction.

The half-life of alanine aminotransferase in rat muscle is about 20 days, compared with that of the same enzyme in liver of about three days. The thermal denaturation rate of the enzyme in , vitro gives a calculated half-life of about 400 days at body temperature. A liver lysosomal preparation effectively inactivated the enzyme under conditions in which it was otherwise quite stable. From our results we conclude that turnover of alanine aminotransferase in , vivo is a reflection of an active (enzymatic) process which does not depend on prior thermal denaturation.

It is well known that ornithine-ketoacid transaminase(OKT) catalyzes the interconversion of ornithine and gluaamic-δ-semialdehyde with concomitant interconversion of α-ketoglutarate and glutamate in animal tissues. In 1963 Peraino and Pitot reported some properties of OKT using partially purified enzyme. In 1964 we reported the purification of the enzyme to a single component by ultracentrifugal and electrophoretic analysis. Strecher also reported some important kinetical properties using partially purified enzyme. Recently, we purified and crystallized the enzyme in good yiels from rat liver. This is reported in this paper with some physicochemical properties of OKT.