Validation of the diagnostic value of plasma pyridoxal 5'-phosphate measurements in vitamin B6 nutrition of the rat.

The relationship of plasma pyridoxal 5'-phosphate (PLP) to PLP content of tissues and activities of PLP-dependent enzymes was examined to establish its value in assessing vitamin B6 nutrition. Weanling rats were fed ad libitum for 9 weeks purified diets which supplied 0, 4, 12, 24, and 100 micrograms of pyridoxine daily. Growth increased with increasing pyridoxine intake, reaching a maximum at 24 micrograms/day. Liver and brain PLP also increased, attaining maximal values at 12 micrograms. By contrast, muscle and plasma PLP did not saturate when vitamin B6 intake was increased to 100 micrograms. Erythrocytic holoenzyme activity of aspartate (Asp) aminotransferase became maximal with 24 micrograms but that of alanine (Ala) aminotransferase did not. Hepatic holoenzyme activities of Ala, Asp and tyrosine aminotransferases reached maximal values with only 4 micrograms vitamin B6 but that of serine dehydratase became maximal with 12 micrograms. Measurement of coenzyme saturability suggested that apoenzyme degradation, coenzyme affinity and PLP transfer determine the activities of these enzymes. It is concluded that plasma and muscle PLP behave as mobilizable storage pools and that plasma PLP is a sensitive and reliable indicator of vitamin B6 nutrition.