ORNITHINE-DELTA-AMINOTRANSFERASE IN LACTATING BOVINE MAMMARY-GLANDS

The occurrence and subcellular distribution of ornithine-delta-aminotr ansferase have been studied in lactating bovine mammary glands. The en zyme is localized in the mitochondria and has a unique thermal reactio n profile that distinguishes it from putative liver and kidney isozyme s. The enzyme concentration in the gland correlates well with a role i n the conversion of ornithine into the proline precursor, L-Delta(1)-p yrroline-5-carboxylate. However, an unusually high Michaelis constant for the mitochondrial enzyme (8.4 mM) raises the question of enzyme ef ficiency in vivo such that this pathway needs to be considered in esti mating barriers to protein secretion into milk.