electron-paramagnetic-resonance studies using oxygen-17

Studies of the effect of substitution with 170 on the e.p.r. spectra at 9 and 35 GHz of Mo(V) in the phosphate complex of sulphite oxidase are reported. Substitution of '7O-enriched water for normal water, for samples of the enzymes reduced by sulphite in the presence of normal phosphate, produced no detectable effect on the e.p.r. signal. If phosphate substituted with 170 was used, coupling due to 170, producing large anisotropic splittings in the spectrum, was clearly detectable. It is concluded that phosphate is co-ordinated directly to molybdenum in the active site of the enzyme, in