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.