It is now well-known that conventional cytochrome P-450s catalyze hydroxylation reactions using an iron mono-oxygen species, the structure of which, as inferred from chemical model studies, may be drrepresented by the following canonical forms: FeV==O<-->(.+)FeIV==O<-->FeIV--O(.). Certain multifunctional P-450s, notably those involved in steroid biosynthesis, catalyze, in addition to hydroxylation reactions, an acyl-carbon cleavage process in which the participation of an iron peroxide intermediate, FeIII--OOH, has been suggested. However the possibility still exists that the C--C bond cleavage may also occur using the FeV==O species. We have scrutinized the chemical consequences of involving either an FeV==O or an FeIII--OOH species for five different C--C bond cleavage reactions. With respect to the status as well as the origin of hydrogen and oxygen atoms, in four of the examples the mechanism involving the FeV==O species makes the same prediction as that using the iron peroxide intermediate, that is, the incorporation of an atom of oxygen from O2 into acyl part of the cleaved fragment. The fifth example, however, involving the formation, with pig testes microsomes, of 17 alpha-hydroxyandrogen (androst-5-ene-3 beta,17 alpha-diol) from pregnenolone, presents an interesting contrast--in this case different outcomes are predicted by the two mechanisms. These possibilities have been experimentally evaluated using substrates stereo- and regiospecifically labeled with heavy isotopes and incubated with pig testes microsomes under either 16O2 or 18O2.(ABSTRACT TRUNCATED AT 250 WORDS)