Metabolism of Arginine and Ornithine in the Cow and Rabbit Mammary Tissue

Abstract

Amino acid uptake by the bovine mammary gland was determined by arteriovenous difference. Extraction of arginine from the plasma by the lactating bovine mammary gland was in excess of requirements for milk protein synthesis. Ornithine and citrulline also were extracted by the gland but are not in milk protein. Incubations of slices of lactating mammary tissue from cows and rabbits indicate that nonessential amino acids, especially proline and glutamate, are the major end products of arginine and ornithine metabolism in the lactating mammary gland.

Arginine metabolism: nitric oxide and beyond

“…The output of proline in the milk of goats [255], sheep [256], cows [257] and pigs [258] greatly exceeds the uptake of proline by the lactating mammary gland, whereas the uptake of plasma arginine by lactating mammary glands greatly exceeds the output of arginine in the milk [255][256][257][258][259]. Studies with lactating mammary tissues have demonstrated arginine-dependent production of proline [249,257], but there was little or no synthesis of proline from glutamate [249] because of the absence of P5C synthetase [47,249]. Uptake of ornithine and citrulline (potential precursors for proline synthesis) by the lactating mammary gland is relatively low compared with that of arginine [256,257].…”

“…Studies with lactating mammary tissues have demonstrated arginine-dependent production of proline [249,257], but there was little or no synthesis of proline from glutamate [249] because of the absence of P5C synthetase [47,249]. Uptake of ornithine and citrulline (potential precursors for proline synthesis) by the lactating mammary gland is relatively low compared with that of arginine [256,257]. There is virtually no proline catabolism by the mammary gland because it lacks proline oxidase [249].…”

“…As the high content of these amino acids in milk greatly exceeds their accumulation via uptake by the lactating mammary gland [258,265], they must be synthesized within this organ. As in the case of proline synthesis during lactation, arginase also plays an important role in providing substrate for glutamate synthesis in the mammary gland via type II arginase, OAT and P5C dehydrogenase [249,257]. Although ornithine is used for the synthesis of both proline and glutamate in all stages of lactation, it is preferentially used for synthesis of glutamate plus glutamine in the later stages of lactation [249].…”

Proline Supply for Efficient Nitrogen Utilization by Lactating Ruminants

“…Cow mammary tissue was cultured in media with increasing concentrations of proline. (Clark et al 1975;Verbeke et al 1968). This process is connected with a substantial loss of nitrogen.…”

“…tein commonly supplied to the duodenum (up to 5Va) by the usual diets, -207o of the proline needed for casein synthesis is synthesized from arginine and ornithine (Clark et al 1975;Verbeke et al 1968). This process is connected with a substantial loss of nitrogen.…”