In our previous report we showed cytochrome b5 to be a competitive inhibitor of cAMP-dependent protein kinase (PKA) for interaction with cytochrome P450 (P450). While P450 was phosphorylated, cytochrome b5 was not. The phosphorylation of P450 resulted in an inhibition of its catalytic activity. In this report we attempt to determine the relationship between phosphorylation of P450 from phenobarbital-induced rat and its destruction. The results indicate there is a considerable alteration of P450 IIB1 when it is put into the phosphorylation medium. This includes destruction, i.e., loss of the hemoprotein nature (Soret peak), as well as denaturation, conversion of a proportion of the P450 to P420. The extent of phosphorylation correlated best with the amount of destroyed hemoprotein, and not with the formation of P420. There did not appear to be phosphorylation-dependent formation of apo-P450. Further, prior conversion of the P450 to P420 using sodium deoxycholate showed the same extent of phosphorylation as before the conversion. Thus, intact P450 is not required for phosphorylation nor is phosphorylation a prerequisite for hemoprotein destruction. P450 CAM (CIA1), which has the PKA substrate recognition sequence internalized, likewise undergoes conversion to P420 but this denaturation does not result in phosphorylation. Destruction of CIA1 with 6 M urea, however, did permit phosphorylation by PKA. P450 IIB1 destruction was greatly diminished by cytochrome b5. This stabilization resulted in a decreased degree of phosphorylation as well as an increase in negative ellipticity in circular dichroism, indicative of an increase in the proportion of alpha-helical content in the P450. Suggestions are made that this structural modification caused by cytochrome b5 stabilizes the P450 against denaturation as well as against destruction and phosphorylation. Further, when the P450 IIB1 was kept stable as P450 in the absence of cytochrome b5 and without loss of hemoprotein during the incubation period, using phosphate-glycerol buffer containing 0.4% Emulgen 911, the phosphorylation of the P450 was greatly diminished, with only minor effects on the protein kinase reaction itself. These results suggest that the protein kinase reaction itself. These results suggest that the protein kinase substrate recognition sequence is not readily accessible to PKA in mammalian P450 IIB1 but requires a destabilization of the protein for phosphorylation to take place.