Consumption of vitamin B12 during sodium nitroprusside administration in humans

In view of evidence suggesting possible participation of cobalamin in cyanide metabolism, the effects of sodium nitroprusside (SNP) infusion on blood cyanide and plasma thiocyanate levels and serum vitamin B12 levels were examined in ten patients undergoing major orthopedic procedures. Whole blood cyanide concentrations increased significantly (P less than 0.001) from 3.6 +/- 1.1 to 65.7 +/- 16.2 micrograms/dl, and total serum B12 values decreased significantly (P less than 0.005) from 482 +/- 56 to 267 +/- 42 pg/ml, three hours after SNP therapy. Plasma thiocyanate did not change. Cyanide released from SNP converts hydroxocobalamin to cyanocobalamin, which is readily excreted in urine. These variables were not changed in six patients (control) who received trimethaphan. In vitro experiments revealed that none of the serum factor(s) nor nitroprusside interfered with the assay of B12. Hydroxocobalamin may be an appropriate adjunct during SNP therapy.