Dietary protein quality alters ornithine decarboxylase activity but not vitamin B-6 nutritional status in rats

Weanling male rats were fed diets that varied in protein quality (casein or wheat gluten) and vitamin B-6 (0.0, 0.5, 1.0 and 1.5 mg pyridoxine HCl/kg diet) to test the hypotheses that low protein quality would depress vitamin B-6 nutritional status and that activity of ornithine decarboxylase (ODC) would be a sensitive functional indicator of vitamin B-6 nutritional status. The wheat gluten diet depressed body weight gain approximately 17% at higher vitamin B-6 levels, as expected. However, vitamin B-6 nutritional status was not worse in gluten-fed compared with casein-fed groups, as evidenced by static measures (B-6 vitamer concentrations in plasma and tissues) and a functional indicator (tryptophan load test). The activity of ODC (holo- and total) in liver, kidney and small intestine did not vary significantly at the three higher levels of vitamin B-6 intake. In groups fed casein, total ODC activity in these tissues was two- to fivefold higher in rats fed diets containing 0.0 mg vitamin B-6/kg compared with higher B-6 levels, without corresponding differences in ODC mRNA abundance in liver and kidney. Concentrations of B-6 vitamers (except pyridoxal phosphate in plasma) increased linearly with dietary vitamin B-6 in plasma, liver, kidney and intestine. These data suggest that low quality protein fed as wheat gluten suppresses growth but not vitamin B-6 nutritional status, and that ODC activity is not a sensitive functional indicator of marginal vitamin B-6 status.