The effect of glycerol on cytochrome P450scc (CYP11A1) spin state, activity, and hydration

We examined the effects of glycerol, a stabilizing agent commonly used in cytochrome P450scc purification and analysis, on the spin state, catalytic activity, and molecular volume of the cytochrome. Glycerol induced a sigmoidal low-spin response. The binding of hydroxycholesterol reaction intermediates, but not cholesterol, increased the concentration of glycerol required for the spin transition to be 50% complete (K1/2). Glycerol weakened adrenodoxin binding to P450scc but had no effect on CO or 20α,22R-dihydroxycholesterol binding. Cytochrome P450scc activity was inhibited by glycerol with the K1/2 for inhibition being substrate-dependent. The osmotic stress exerted by glycerol on P450scc resulted in decreases in P450scc molecular volume for both the transition to low spin state and the inhibition of activity. From this we determined that two dissociative water molecules are involved in the inhibition of activity with cholesterol as substrate and five or six dissociative waters are involved in the low-spin transition. The dehydration of P450scc by osmotic stress provides an explanation for the effects of glycerol on P450scc spin transition and activity.