Mass spectrometric analysis of rabbit and bovine trypsin-solubilized cytochrome b5.

The sequence and blocking group of the amino-terminal 15 amino acids of rabbit trypsin-solubilized cytochrome b5 were determined by liquid secondary ion mass spectrometry (LSIMS) and tandem mass spectrometry (MS/MS). The molecular weights of peptides generated from a Staphylococcus aureus V8 protease digest of this protein were determined by LSIMS analysis and the two peptides containing the blocked amino-terminus were sequenced by tandem mass spectrometry to yield the sequence; N-acetyl-Ala-Ala-Glu-Ser-Asp-Lys-Asp-Val-Lys-Tyr-Tyr-Thr-Leu-Glu-Glu. Comparison of this sequence with a recently reported cDNA sequence (Dariush et al., 1988) indicates that Gln at position 3 is selectively deamidated, although no other discrepancies were found. Intact rabbit and bovine trypsin-solubilized cytochrome b5 were also analyzed by LSIMS on a high-field mass spectrometer equipped with a diode array detector. Mass measurement of the unresolved protonated molecular ion peak tops gave average molecular weights of 9462.2 +/- 2 and 9502.3 +/- 2 for bovine and rabbit trypsin-solubilized cytochrome b5, respectively. In both cases, these molecular weights correspond to a cytochrome b5 fragment consisting of amino acids Asp(7)-Arg(88). The average molecular weight for the rabbit amino-terminal-blocked form of trypsin-solubilized cytochrome b5 was found to be 10,144.5 +/- 2, which was consistent with the molecular weight predicted for the extended N-acetylated form (residues 1-88) of Mr 10,146.1.

Methoxyflurane acts at the substrate binding site of cytochrome P450 LM2 to induce a dependence on cytochrome b5.

Rabbit cytochrome P450 isozyme 2 requires cytochrome b5 to metabolize the volatile anesthetic methoxyflurane but not the substrate benzphetamine [E. Canova-Davis and L. Waskell (1984) J. Biol. Chem. 259, 2541-2546]. To determine whether the requirement for cytochrome b5 for methoxyflurane oxidation is mediated by an allosteric effect on cytochrome P450 LM2 or cytochrome P450 reductase, we have investigated whether this anesthetic can induce a role for cytochrome b5 in benzphetamine metabolism. Using rabbit liver microsomes and antibodies raised in guinea pigs against rabbit cytochrome b5, we found that methoxyflurane did not create a cytochrome b5 requirement for benzphetamine metabolism. Methoxyflurane also failed to induce a role for cytochrome b5 in benzphetamine metabolism in the purified, reconstituted mixed function oxidase system. Studies of the reaction kinetics established that in the absence of cytochrome b5, methoxyflurane and benzphetamine are competitive inhibitors, and that in the presence of cytochrome b5, benzphetamine and methoxyflurane are two alternate substrates in competition for a single site on the same enzyme. These results all indicate that the methoxyflurane-induced cytochrome b5 dependence of the mixed function oxidase cytochrome P450 LM2 system is a direct result of the interaction between methoxyflurane and the substrate binding site of cytochrome P450 LM2 and suggest the focus of future studies of this question.

A gas chromatographic mass spectrometric method for the analysis of trifluoroacetic acid: application to the metabolism of halothane by in vitro preparations.

A selected ion monitoring gas chromatographic mass spectrometric assay for trifluoroacetic acid was developed for the study of the metabolism of the volatile anesthetic agent halothane by in vitro preparations. The assay uses a headspace sampling technique after formation of methyl esters with dimethyl sulfate and sulfuric acid. Pentafluoropropionic acid proved to be a suitable internal standard, although care is required in the preparation of the calibration standards so that they reflect the composition and treatment of the samples. Contamination of the samples, possibly with trifluoroacetic acid itself, was found to be the primary factor in limiting the sensitivity of the assay for the metabolism of halothane. Generally, trifluoroacetic acid could be determined at levels as low as 1 microM in 50-100 microliters of incubate.

Characterization of halothane oxidation by hepatic microsomes and purified cytochromes P-450 using a gas chromatographic mass spectrometric assay.

A sensitive assay for trifluoroacetic acid, the major product of the oxidative metabolism of halothane, has been developed to study the biotransformation of halothane. A selected ion monitoring gas chromatographic mass spectrometric assay measured trifluoroacetic acid levels as low as 1 microM in 100 microliter of reaction mixture. This assay was used to quantitate halothane metabolism in human and rabbit microsomal systems and with purified proteins. Trifluoroacetic acid production was examined as a function of the concentration of substrate present, the amount of microsomal protein used and the length of reaction time. Halothane metabolism in microsomes was linear for at least 30 min, and up to a microsomal protein concentration of 1 mg/ml. In rabbits, phenobarbital and imidazole induced the microsomal metabolism of halothane 7.36- and 18.2-fold, respectively. Imidazole was used because it is a potent inducer of cytochrome P-450 isozyme 3a which is also induced by ethanol. The cytochrome P-450 in microsomes from a single human subject metabolized halothane at a rate comparable to that found in microsomes from phenobarbital- and imidazole-pretreated rabbits. The purified phenobarbital and imidazole inducible cytochromes P-450, isozymes 2 and 3a, catalyzed the oxidation of halothane to trifluoroacetic acid. Cytochrome b5 stimulated the isozyme 3a-catalyzed oxidation of halothane by 19-fold, whereas isozyme 2 catalyzed oxidation was increased 4.3-fold. Antibodies to cytochrome P-450 3a inhibited halothane metabolism by 90% in microsomes from imidazole-pretreated rabbits, suggesting that isozyme 3a catalyzes halothane metabolism in imidazole-pretreated rabbits. In conclusion, the oxidation of halothane to trifluoroacetic acid by cytochrome P-450 isozymes 3a and 2 is enhanced markedly by cytochrome b5.