Alkaline phosphatase activity and pyridoxal phosphate concentrations in the milk of various species

Because pyridoxal phosphate does not normally cross membranes, it was intriguing that the concentration of pyridoxal phosphate is much higher in goat milk than in human milk. We also noted that, although the total vitamin B-6 concentration of bovine milk was similar to that of caprine milk, the bovine milk had lower pyridoxal phosphate. Preliminary data from five Alpine goats, five Brown Swiss cows, five Holstein cows and three humans suggested that there was an inverse relationship between pyridoxal phosphate concentration and phosphatase activity in the goats and cows but not in the humans. This was confirmed with additional data from Nubian goats, Jersey and Guernsey cows, and crossbred sows. Combining the animal data yielded the following relationship between pyridoxal phosphate (PLP, mumol/L) and alkaline phosphatase (P'ase) activity (mmol/(min.L): PLP = 2.03e(-2.26 P'ase) + 0.03. The human milk samples were low in both pyridoxal phosphate and alkaline phosphatase. We conclude that in goats, cows and pigs a significant fraction of the vitamin B-6 appearing in the milk is secreted as pyridoxal phosphate, probably bound to protein, and varying amounts may then be hydrolyzed back to pyridoxal depending on the alkaline phosphatase activity. Human mammary tissue apparently secretes very little pyridoxal phosphate.