Microheterogeneity of cytochrome P450(LM2) as revealed by isoelectric focusing

The homogeneity of different preparations of cytochrome P450(LM2), the major form of cytochrome P450 found in the liver microsomes of phenobarbital-treated rabbits, was checked by SDS polyacrylamide gel electrophoresis and N-terminal amino acid sequencing. In SDS-PAGE all the preparations migrated as a single protein band of molecular mass 49 kDa. The electrophoretic mobility of this band corresponded to that of the major phenobarbital-inducible band in the microsomes of phenobarbital-treated rabbits. A single N-terminal sequence (Met-Glu-Phe) corresponding to the N-terminal sequence of the LM2 form of cytochrome P450 was revealed by N-terminal amino acid sequence analysis. Isoelectric focusing of electrophoretically homogeneous cytochrome P450(LM2) was conducted in polyacrylamide gel in an LKB Ampholine pH 5-8 gradient under denaturing conditions (9.1 M urea). Under these conditions cytochrome P450(LM2) was resolved into six subfractions in the pH range 6.5-7.75. The data obtained reflect the microheterogeneity of this form of cytochrome P450. The existence of different isoelectric points cannot have been due to different protein conformations because denaturing conditions were used, but seem to reflect different primary structures of the cytochrome subfractions.