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Validation of the Diagnostic Value of Plasma Pyridoxal 5'-Phosphate Measurements in Vitamin B₆ Nutrition of the Rat^1,2,

Indiana University School of Medicine, Departments of Medicine and Biochemistry and the Veterans Administration Hospital, Indianapolis, Indiana 46202

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 µg of pyridoxine daily. Growth increased with increasing pyridoxine intake, reaching a maximum at 24 µg/day. Liver and brain PLP also increased, attaining maximal values at 12 µg. By contrast, muscle and plasma PLP did not saturate when vitamin B₆ intake was increased to 100 µg. Erythrocytic holoenzyme activity of aspartate (Asp) aminotransferase became maximal with 24 µg but that of alanine (Ala) aminotransferase did not. Hepatic holoenzyme activities of Ala, Asp and tyrosine aminotransferases reached maximal values with only 4 µg vitamin B₆ but that of serine dehydratase became maximal with 12 µg. 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.

KEY WORDS: • plasma pyridoxal 5'-phosphate • tissue pyridoxal 5'-phosphate • vitamin B₆ nutrition • B₆-dependent enzymes

¹ Supported by a grant from the Veterans Administration Hospital (MRIS 583-5246).

² Part of this work was presented at the 60th Annual Meeting of the Federation of American Societies for Experimental Biology, Anaheim, California, April 1976. Federation Proc. 35, 660 (1976).

³ To whom reprint requests should be sent.

Manuscript received 21 June 1977.