Rat liver L-threonine dehydrogenase.

L-threonine dehydrogenase (E.C.1.1.1.103) catalyzes the NAD+-dependent transformation of Lthreonine to 2-amino 3-0x0-butyric acid, which spontaneously loses CO2 with the final production of aminoacetone ( 1) The enzyme is present in several microorganisms as well as in the liver of vertebrates (2-6). We studied some properties of the rat liver enzyme, showing that the activity was strongly inhibited by an unknown "factor" in the mitochondrial preparations, easily dialyzable and not absorbed by norite or extracted by acetone or other solvents treatments. On the contrary, it precipitated with barium treatment and was also alkali and acid resistent. We studied the behaviour of the enzyme during the storage of rat liver mitochondria at -20°C. Enzymatic activity increased, reaching a maximum after 2 weeks. storage, the activity further increased. we used male albino Wistar rats. The mitochondria1 fractions were prepared according to De Duve (7). The determination of dehydrogenase activity was run out as previously described ( 8 ) . In this study we investigated the pH optimum of the enzyme in three different preparations: i) fresh mitochondria; ii) mitochondria frozen for 14 days; iii) mitocondria frozen for 14 days and dialyzed. We also studied the effects of bicarbonate, tris, and phosphate buffers exerted on the rat liver dehydrogenase activity. The results are reported in Fig. 1: a) there was a net increase in activity after freezing