ARGINASE IN LACTATING BOVINE MAMMARY-GLANDS - IMPLICATIONS IN PROLINESYNTHESIS

The occurrence and subcellular distribution of arginase have been stud ied in mammary glands from lactating dairy cattle. The enzyme appears to be localized in the mitochondrial fraction, although a significant amount has been found to be associated with the cytosolic fraction. Bo th mitochondrial and cytosolic arginase are activated by heating with Mn2+. The Michaelis constants for the two fractions, however, are diff erent: 49.5 and 18.5 mM for the mitochondrial fraction and cytosolic f raction, respectively. Overall, the total enzyme concentration in the gland suggests that these enzymes contribute to the conversion of argi nine to ornithine. Ornithine, in turn, may be converted by ornithine a minotransferase into an intermediate, L-Delta(1)-pyrroline-5-carboxyla te; concurrently, alpha-keto-glutarate is transformed into glutamic ac id. Finally, pyrroline-5-carboxylate reductase yields proline, an impo rtant amino acid that is needed for casein synthesis. Because pyrrolin e-5-carboxylate reductase requires NADPH, and because ornithine aminot ransferase uses alpha-ketoglutarate, this, new pathway is linked to th e Krebs cycle through the cytosolic isocitrate dehydrogenase, which is the source of both of these intermediates.