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 e nzyme appears to have only a cursory association with the mitochondria l fraction, because significant amounts of the enzyme are found in oth er membrane-containing fractions and in the cytosol. Polyamines stimul ate the enzyme in vitro, supporting the mediation of cursory attachmen t to membrane fractions by these compounds. The enzyme is selective fo r NADPH but can utilize NADH as well. Long-chain coenzyme A derivative s, which are generated during lipid metabolism, almost completely inhi bit this enzyme, which is responsible for the synthesis of a portion o f the proline needed for casein production. Overall, the enzyme concen tration 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 ca sein synthesis.