Validation of the Diagnostic Value of Plasma Pyridoxal 5′-Phosphate Measurements in Vitamin B6 Nutrition of the Rat

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Abstract
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.

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