Vitamin B6 metabolites in idiopathic calcium stone formers: no evidence for a link to hyperoxaluria

Vitamin B6 metabolites and their potential correlates to urinary oxalate excretion in idiopathic calcium stone formers (ICSF) compared with healthy subjects were investigated. This clinical study was performed in a population of male ICSF with (Hyperoxalurics, n=55) or without hyperoxaluria (Normooxalurics, n=57) as well as in 100 healthy male control subjects. Pyridoxal 5-phosphate serum concentration (S-pyridoxal 5P) and 24-h urinary excretion of 4-pyridoxic acid (U-4pyridoxic acid) were measured using HPLC; 24-h urinary excretion of oxalate (U-oxalate) was measured concurrently. A subgroup of subjects (40 Hyperoxalurics, 15 Normooxalurics and 50 controls) underwent the same measurements before and after 7-day pyridoxine loading per os (pyridoxine hydrochloride, 300 mg/d). Under usual conditions, U-4pyridoxic acid was similar in the three groups, whereas mean S-pyridoxal 5P was significantly lower (p<0.0001) in the Hyperoxalurics (59.6±21.2 nmol/L) and in the Normooxalurics (64.9±19.7 nmol/L) than in the controls (86.0±31.0 nmol/L). No correlation could be found between U-oxalate and U-4pyridoxic acid or S-pyridoxal 5P. After B6 loading, S-pyridoxal 5P was still significantly lower in the Hyperoxalurics (415±180 nmol/L, p<0.001) and in the Normooxalurics (429±115 nmol/L, p=0.036) than in the controls (546±180 nmol/L), although there was no difference between groups for U-4pyridoxic acid. No correlation in any group could be found between changes in U-oxalate and changes in U-4pyridoxic acid or S-pyridoxal 5P. Although there is no vitamin B6 deficiency in ICSF with or without hyperoxaluria, these patients, on average, have lower levels of S-pyridoxal 5P than healthy subjects. However, this slight decrease does not seem to account for idiopathic hyperoxaluria.