Purification of a human liver cytochrome P-450 immunochemically related to several cytochromes P-450 purified from untreated rats.

Among characterized forms of liver microsomal cytochromes P-450 in rats are four related isozymes (P-450f-i) notable for their lack of inducibility. Immunoblot analyses demonstrated that human livers microsomes contained several proteins related to these rat P-450s. A human liver P-450, termed HLx, was purified and found by immunochemical assays to resemble rat P-450g. Analysis of the NH2-terminal amino acid sequence of HLx indicates that it is related to rat P-450s f-i and human liver P-450MP. A monoclonal antibody was used to measure the amounts of HLx in 21 human liver specimens. No correlation between the levels of HLx protein in these specimens and the patients' environmental histories was observed. However, statistical analysis of the data suggests that the distribution of HLx is at least bimodal. We conclude that HLx is a member of a family of human liver P-450s that resembles in its structure, and possibly in its distribution, several liver P-450s found in other animals.

A simple, non-chromatographic purification procedure for monoclonal antibodies. Isolation of monoclonal antibodies against cytochrome P450 isozymes.

A simple, non-chromatographic purification procedure for monoclonal antibodies from mouse ascites fluid is described. This procedure, which is rapid, inexpensive, and has high capacity involves the precipitation of contaminating proteins with caprylic acid followed by precipitation of immunoglobulin using ammonium sulfate. This two-step procedure is shown to be effective for the purification of various immunoglobulins including IgG1, IgG2a and IgG2b. In the present report, more than 30 monoclonal antibodies directed against cytochrome P450 isozymes have been purified by this method and characterized.

Monoclonal antibodies distinguish among isozymes of the cytochrome P-450b subfamily.

Polyclonal antibody has been shown previously to react identically with cytochromes P-450b and P-450e purified from Long Evans rats and a strain variant of cytochrome P-450b purified from Holtzman rats (P-450bH). In the present study, an array of 12 different monoclonal antibodies produced against cytochrome P-450b has been used to distinguish among these closely related phenobarbital-inducible rat hepatic cytochromes P-450. In immunoblots and enzyme-linked immunosorbent assays, 10 monoclonal antibodies bind to cytochromes P-450b, P-450e, and P-450bH; one monoclonal antibody (B50) recognizes cytochromes P-450b and P-450bH but not cytochrome P-450e; and one monoclonal antibody (B51) is specific for cytochrome P-450b. In addition, one monoclonal antibody (BEF29) reacts strongly with cytochrome P-450f, and another antibody (BEA33) reacts weakly with cytochrome P-450a. No cross-reactions with cytochromes P-450c, P-450d, and P-450g-P-450j were detected with any of the monoclonal antibodies in these assays. Six spatially distinct epitopes on cytochrome P-450b were identified, and differences in antibody reactivity provided evidence for three additional overlapping epitopes. Several monoclonal antibodies are potent inhibitors of testosterone and benzphetamine metabolism supported by cytochrome P-450b in a reconstituted system. B50 and BE52 do not inhibit metabolism of the two substrates by microsomes from untreated rats, but inhibit benzphetamine N-demethylation and testosterone metabolism to 16 alpha- and 16 beta-hydroxytestosterone as well as androstenedione formation 67-94% by microsomes from phenobarbital-treated rats. No other pathways of testosterone metabolism are inhibited by these monoclonal antibodies. The differential inhibition of microsomal metabolism of benzphetamine and testosterone by these monoclonal antibodies is a reflection of the content and inducibility of cytochromes P-450b and P-450e as well as other cytochrome P-450 isozymes.

Regulation of cytochrome P-450j, a high-affinity N-nitrosodimethylamine demethylase, in rat hepatic microsomes.

Polyclonal antibodies were produced in rabbits against purified cytochrome P-450j isolated from isoniazid-treated adult male rats. The monospecificity of immunoadsorbed antibody to cytochrome P-450j was demonstrated by Ouchterlony double diffusion analyses, enzyme-linked immunosorbent assays, and immunoblots. Immunoquantitation results indicated that rat liver microsomal cytochrome P-450j content decreases between 3 and 6 weeks of age in both the male and female animal. Several xenobiotics, such as Aroclor 1254, mirex, and 3-methylcholanthrene, repressed cytochrome P-450j levels when administered to male rats. Isoniazid, dimethyl sulfoxide, pyrazole, 4-methylpyrazole, and ethanol were inducers of cytochrome P-450j in rat liver although these compounds showed different inducing potencies. Microsomes from adult male rats with chemically induced diabetes also contained elevated levels of cytochrome P-450j compared to untreated animals. Cytochrome P-450j levels were measurable in kidney, whereas this isozyme was barely detectable in lung, ovaries, and testes; however, extrahepatic cytochrome P-450j was inducible by isoniazid. Approximately 80-90% of microsomal N-nitrosodimethylamine demethylation was inhibited by antibody to cytochrome P-450j whether the microsomes were isolated from untreated rats or animals administered inducers or repressors of cytochrome P-450j. The residual catalytic activity resistant to antibody inhibition may be a reflection of the inaccessibility of a certain amount of cytochrome P-450j due to interference by NADPH-cytochrome P-450 reductase based on results obtained with the reconstituted system. There was a good correlation (r2 = 0.87) between cytochrome P-450j content and N-nitrosodimethylamine demethylase activity in microsomes from rats of different ages and treated with various xenobiotics. The evidence presented indicates that cytochrome P-450j is the primary, and perhaps sole, microsomal catalyst of N-nitrosodimethylamine demethylation at substrate concentrations relevant to hepatocarcinogenesis induced by N-nitrosodimethylamine.

Monoclonal antibodies of differentiating specificities as probes of cytochrome P450h (2C11).

A panel of 30 monoclonal antibodies against rat hepatic microsomal cytochrome P450h (2C11) has been produced, purified, and characterized. A broad range of reactivities was observed when 13 purified rat cytochrome P450 isozymes were tested for epitope relatedness in a noncompetitive enzyme-linked immunosorbent assay or on immunoblots. Several antibodies were antigen-specific, others reacted with additional members of the 2C subfamily, and other monoclonal antibodies recognized cytochromes P450 from the 2E, 2B, 2A, and 1A subfamilies. Cytochromes P450p (3A1) and P4501 (3A2) did not react with any of the antibodies. A minimum of seven spatially distinct epitopes on cytochrome P450h were defined by the panel of antibodies. Immunoblot analysis of rat microsomes illustrated the male specificity of cytochrome P450h expression which extended to extrahepatic tissues including kidney and lung. A survey of various species by immunoblot analysis with several antibodies revealed little if any epitope relatedness among microsomal proteins from rats, mice, rabbits, hamsters, squirrel monkeys, guinea pigs, or humans. All of the antibodies were screened as potential inhibitors of cytochrome P450h-mediated testosterone hydroxylation in a reconstituted system. Although most of the antibodies were noninhibitory, greater than 70% inhibition of 2 alpha- and 16 alpha-hydroxylation of testosterone was observed with selected antibodies. These inhibitory antibodies gave similar results when benzphetamine N-demethylation was evaluated in the reconstituted system. The inhibitory antibodies were then used to assess the role of cytochrome P450h in microsomal benzphetamine N-demethylation, since this isozyme exhibits high catalytic activity for this substrate. Only 20-25% inhibition of benzphetamine metabolism was attained in microsomal preparations from adult male rats, and the antibodies did not influence the microsomal catalytic activity of immature males or females or adult females. Thus, despite the high level of expression of cytochrome P450h in microsomes from adult male rats and the high catalytic activity of the purified protein for benzphetamine, this isozyme contributes only a small portion of the metabolism of this substrate in microsomes.

Polyclonal and monoclonal antibodies as probes of rat hepatic cytochrome P-450 isozymes.

Cytochrome P-450 is the terminal oxidase of an electron transport system that is responsible for the oxidative metabolism of a large variety of endogenous and exogenous compounds. This broad substrate selectivity is caused by multiple isozymes of cytochrome P-450 and the wide substrate selectivity of many of these isozymes. We have isolated 11 isozymes of cytochrome P-450 from the livers of rats (cytochromes P-450a-P-450k). We have found both polyclonal and monoclonal antibodies increasingly useful to distinguish among these isozymes and to quantitate enzyme levels in liver microsomal preparations where as many as 15 or more cytochrome P-450 isozymes are present. Several of these isozymes show considerable immunochemical relatedness to each other, and operationally they can be grouped into families of immunochemically related isozymes that include cytochromes P-450b and P-450e in one family, cytochromes P-450c and P-450d in another, and cytochromes P-450f-P-450i, and P-450k in a third family. Immunoquantitation of some of these isozymes has revealed dramatic increases of over 50-fold in the levels of certain of these isozymes when exogenous compounds are administered to rats.

Identification of a human liver cytochrome P-450 homologous to the major isosafrole-inducible cytochrome P-450 in the rat.

The rat 3-methylcholanthrene-inducible family of liver cytochromes P-450 contains two proteins (P-450c and P-450d) that are immunochemically related, possess 68% total sequence homology, and are induced by a number of toxic or carcinogenic compounds. To determine whether equivalent isozymes of hepatic cytochrome P-450 are expressed in humans, as they are in several mammalian species, we performed immunoblot analyses on microsomes prepared from 14 human liver specimens and found that each one contained a 52.5-kDa protein (termed HLd) that reacted with antibodies specific for rat P-450d. In addition, one specimen contained a 54-kDa protein (termed HLc) that reacted with antibodies specific for rat P-450c. HLd was purified through the use of immunoaffinity chromatography and was found to be 56% homologous to rat P-450d and 61% homologous to the equivalent isozyme in the rabbit (P-450 LM4) through their first 18 NH2-terminal amino acids. Finally, levels of immunoreactive HLd varied more than 10-fold among these patients but were unrelated to the patients' drug treatments, smoking habits, or amount of immunoreactive HLp, a human liver cytochrome P-450 related to the glucocorticoid-inducible family of rat cytochromes P-450. We conclude that, in man, there is a cytochrome P-450 family composed of two isozymes (HLc and HLd) that are immunochemically and structurally related to the 3-methylcholanthrene-inducible family observed in several other species.

Characterization of ethanol-inducible human liver N-nitrosodimethylamine demethylase.

Through the use of monospecific antibodies directed against hepatic cytochrome P-450j, an enzyme induced in rats treated with ethanol or isoniazid, we have purified from human liver the related cytochrome P-450 termed HLj. HLj resembles rat P-450j and P-450 LM3a, the homologous cytochrome in rabbit liver, in its NH2-terminal amino acid sequence, in being in highest concentration in liver microsome samples prepared from two patients intoxicated by ethanol and one patient given isoniazid, and in catalyzing the metabolic activation of the procarcinogen N-nitrosodimethylamine. Furthermore, each of nine human liver RNA samples contained a species of mRNA hybridizable to a cloned HLj cDNA. We conclude that HLj is related by structure, function, and some regulatory characteristics to rat P-450j and rabbit P-450 LM3a, cytochromes critical for metabolism of several clinically relevant cytotoxic and carcinogenic agents.