Vitamin B6 nutritional status in asthma: the effect of theophylline therapy on plasma pyridoxal-5'-phosphate and pyridoxal levels.

Plasma pyridoxal-5'-phosphate concentrations were significantly lower (p less than 0.001) in a group of 28 asthmatic women when compared to 33 controls. Plasma pyridoxal levels in the two groups were not different. Theophylline was administered to a group of 17 volunteers and resulted in large reductions in plasma pyridoxal-5'-phosphate levels, while plasma pyridoxal levels and urinary 4-pyridoxic acid excretion were unaffected by theophylline therapy. An in vitro study showed that theophylline did not interfere with the high performance liquid chromatography assay for pyridoxal-5'-phosphate, indicating that theophylline could affect liver metabolism of vitamin B6.