[Conformation and thermal stability of soluble and liposomal forms of cytochrome P-450]

This secondary structure of soluble cytochrome P-450 and the one incorporated into liposomes from egg lecithin and microsomal lipids has been studied. Using circular dichroism and infrared spectroscopy, it was shown that about 60% of alpha-helices are presented in the structure of haemoprotein and the rest 40% have the structure of statistical coil. The binding of haemoprotein with the type II substrates--octylamine and diaminooctan, slightly increases alpha-helices in soluble cytochrome P-450. The type I non-polar substrates--hexane and cyclohexane--do not change the conformation of isolated enzyme. Cytochrome P-450 incorporated into the artificial membranes of phosphatidyl choline and microsomal phospholipid has almost identical secondary structure as does the soluble one. Data from circular dichroism suggest that the binding of the types I and II substrates to cytochrome P-450 incorporated into lecithin liposomes and microsomal lipid liposomes does not change the conformation of the polypeptide chain. The reduction of cytochrome P-450 haem increases the degree of alpha-spiralization by 10% for soluble haemoprotein and by 5% for the membrane-bound enzyme. The thermal stability of soluble and liposomal forms of cytochrome P-450 was investigated by circular dichroism technique. The effective values of enthalpy and the temperature transition of soluble cytochrome P-450 at pH 6.9, 7,6 and 7,9 are 78, 80 and 78 kcal/mol and 47,7 degrees, 45,2 degrees and 42,4 degrees, respectively. The enzyme incorporated into the phospholipid vesicles is much more stable. The cooperative transition of soluble cytochrome is clearly expressed in contrast to the one of the membrane-bound enzyme.