Effect of Vitamin B6 Depletion in Adult Man on the Excretion of Cystathionine and Other Methionine Metabolites

Abstract

The effect of vitamin B₆ depletion on the metabolism of methionine in six male subjects was studied during a 49-day experiment. When subjects were fed the experimental diet, which contributed 150 g protein and 0.16 mg vitamin B₆ daily, supplemented with 2.00 mg pyridoxine, the average 24-hour pre- and postmethionineloading excretion of cystathionine in the urine was 128 and 163 μmoles, respectively; about 100 μmoles cysteine sulfinic acid were excreted, but no homocystine was found. When the pyridoxine supplement was withheld, within 7 days subjects developed abnormal methionine metabolism as evidenced by an increased excretion of cystathionine following methionine loading. After 3 weeks of vitamin B₆ depletion, urinary cystathionine was markedly increased, pre- and postmethionine values being 1,508 and 3,719 μmoles/24 hours; the respective values for L-cysteine sulfinic acid were 117 and 153 μmoles; the postmethionine value for homocystine was 66 μmoles and trace amounts were detected in the premethionine urines. Repletion with 2.00 mg pyridoxine daily for 2 days caused a sharp decrease in cystathionine excretion, and a 2.00 mg supplement daily for 1 week caused urinary cystathionine, L-cysteine sulfinic acid and homocystine to approach predepletion values. Urinary methionine and taurine were not affected by vitamin B₆ depletion; during the repletion period, however, taurine excretion was significantly less than it was during the predepletion period.