Distribution of adenosine-5'-diphosphatase activity in the lactating bovine mammary gland

Arginase in lactating bovine mammary glands: implications in proline synthesis

The occurrence and subcellular distribution of arginase have been studied 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. Both mitochondrial and cytosolic arginase are activated by heating with Mn2+. The Michaelis constants for the two fractions, however, are different: 49.5 and 18.5 mM for the mitochondrial fraction and cytosolic fraction, respectively. Overall the total enzyme concentration in the gland suggests that these enzymes contribute to the conversion of arginine to ornithine. Ornithine, in turn, may be converted by ornithine aminotransferase into an intermediate, L-delta 1-pyrroline-5-carboxylate; concurrently. alpha-keto-glutarate is transformed into glutamic acid. Finally, pyrroline-5-carboxylate reductase yields proline, an important amino acid that is needed for casein synthesis. Because pyrroline-5-carboxylate reductase requires NADPH, and because ornithine aminotransferase uses alpha-ketoglutarate, this new pathway is linked to the Krebs cycle through the cytosolic isocitrate dehydrogenase, which is the source of both of these intermediates.

Pyrroline-5-carboxylate reductase in lactating bovine mammary glands

The occurrence and subcellular distribution of pyrroline-5-carboxylate reductase have been studied in lactating bovine mammary glands. The enzyme appears to have only a cursory association with the mitochondrial fraction, because significant amounts of the enzyme are found in other membrane-containing fractions and in the cytosol. Polyamines stimulate the enzyme in vitro, supporting the mediation of cursory attachment to membrane fractions by these compounds. The enzyme is selective for NADPH but can utilize NADH as well. Long-chain coenzyme A derivatives, which are generated during lipid metabolism, almost completely inhibit this enzyme, which is responsible for the synthesis of a portion of the proline needed for casein production. Overall, the enzyme concentration in the gland correlates well with a role in the conversion of an intermediate, L-delta 1-pyrroline-5-carboxylate, into proline, an important amino acid for the mammary secretory process, especially casein synthesis.

Ornithine-delta-aminotransferase in lactating bovine mammary glands

The occurrence and subcellular distribution of ornithine-delta-aminotransferase have been studied in lactating bovine mammary glands. The enzyme is localized in the mitochondria and has a unique thermal reaction profile that distinguishes it from putative liver and kidney isozymes. The enzyme concentration in the gland correlates well with a role in the conversion of ornithine into the proline precursor, L-delta 1-pyrroline-5-carboxylate. However, an unusually high Michaelis constant for the mitochondrial enzyme (8.4 mM) raises the question of enzyme efficiency in vivo such that this pathway needs to be considered in estimating barriers to protein secretion into milk.