Glutathione in bovine blood: possible source of amino acids for milk protein synthesis

Glutathione (gamma-glutamyl-cysteinyl-glycine) concentrations of plasma and whole blood were measured for lactating Holstein cows. Samples were taken from both the internal iliac artery and mammary vein to calculate arteriovenous differences across the mammary gland. Glutathione in plasma was extremely low, the there was no significant arteriovenous difference of this tripeptide across the mammary gland. In contrast, glutathione in whole blood was 200-fold greater than in plasma and showed a significant arteriovenous difference across the mammary gland. In vivo studies have shown that when arteriovenous differences of free amino acids in plasma are quantitated, there appears to be a shortage of cysteine for synthesis of milk protein. Using estimated blood flow and arteriovenous differences, we calculated that mammary gland uptake of glutathione from whole blood is more than enough to account for the amount of cysteine secreted in milk. Implications regarding availability to the mammary gland of glutathione from whole blood are discussed.