Monoclonal antibody-directed characterization of rat hepatic P450 catalyzing the omega-1 and omega-2 hydroxylation of prostaglandins

Human CYP2S1 metabolizes cyclooxygenase- and lipoxygenase-derived eicosanoids

CYP2S1 is a recently described dioxin-inducible cytochrome P450. We previously demonstrated that human CYP2S1 oxidizes a number of carcinogens but only via the peroxide shunt. In this article, we investigated whether human CYP2S1 can metabolize cyclooxygenase- and lipoxygenase-derived lipid peroxides in a NADPH-independent fashion. Human CYP2S1 metabolizes prostaglandin G(2) (PGG(2)) (K(m) = 0.267 ± 0.072 μM) into several products including 12S-hydroxy-5Z,8E,10E-heptadecatrienoic acid (12-HHT). It also metabolizes prostaglandin H(2) (PGH(2)) (K(m) = 11.7 ± 2.8 μM) into malondialdehyde, 12-HHT, and thromboxane A(2) (TXA(2)). The turnover to 12-HHT by human CYP2S1 (1.59 ± 0.04 min(-1)) is 40-fold higher than that of TXA(2) (0.04 min(-1)). In addition to PGG(2) and PGH(2) metabolism, human CYP2S1 efficiently metabolizes the hydroperoxyeicosatetraenoic acids (5S-, 12S-, and 15S-) and 13S-hydroperoxyoctadecadienoic acid into 5-oxo-eicosatetraenoic acid (turnover = 16.7 ± 0.3 min(-1)), 12-oxo-eicosatetraenoic acid 1 (11.5 ± 0.9 min(-1)), 15-oxo-eicosatetraenoic acid (16.9 ± 0.8 min(-1)), and 13-octadecadienoic acid (20.2 ± 0.9 min(-1)), respectively. Other cytochromes P450 such as CYP1A1, 1A2, 1B1, and 3A4 underwent similar conversions but at slower rates. The fatty acid hydroperoxides were also converted by human CYP2S1 to several epoxyalcohols. Our data indicate that fatty acid endoperoxides and hydroperoxides represent endogenous substrates of CYP2S1 and suggest that the enzyme CYP2S1 may play an important role in the inflammatory process because some of the products that CYP2S1 produces play important roles in inflammation.

Cytochrome P450-mediated prostaglandin omega/omega-1 hydroxylase activities in porcine ciliary body epithelial cells

The ocular hypotensive effect of topically applied prostaglandins (PGs) is well documented. Although PGs introduced in the posterior chamber accumulate in the anterior tissues (e.g. iris/ciliary complex), little is known about the metabolism of PGs by these tissues. We have recently found that non-pigmented epithelial (NPE) cells and pigmented epithelial (PE) cells are readily separated from porcine ciliary body and cytochrome P450-dependent xenobiotic metabolism is considerably higher in NPE cells than in PE cells. We have therefore investigated in this study the cytochrome P450-mediated PG omega/omega-1 hydroxylase activities of porcine ciliary epithelial cells. The NPE cells show about three times higher activities than do PE cells; the NPE cells, in fact, demonstrate the highest PG omega/omega-1 hydroxylase activities among different ocular tissues. Both omega and omega-1 hydroxylases show broad substrate specificities and hydroxylate PGA1, A2, E1, E2, and lauric acid. The omega/omega-1 hydroxylase activities of NPE and PE, as determined with PGA2 and lauric acid as substrates, are enhanced or induced by treatment of primary cultures of the individual epithelial cells with clofibrate, both activities reaching maximum levels within 48 hr of induction. The induced activities are inhibited almost completely by cycloheximide and actinomycin D. The omega/omega-1 hydroxylase activities of both NPE and PE cells require NADPH and molecular oxygen, are associated with the microsomal fraction, respond to inducers such as clofibrate, and are inhibited by metyrapone and SKF525 A (inhibitors of P450 enzymes). These results support the suggestion that PG omega/omega-1 hydroxylations by NPE and PE are cytochrome P450-mediated reactions.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of cytochrome P450 IA2 in acetanilide 4-hydroxylation as determined with cDNA expression and monoclonal antibodies

The role of P450 IA2 in the hydroxylation of acetanilide was examined using an inhibitory monoclonal antibody (MAb) 1-7-1 and vaccinia cDNA expression producing murine P450 IA1 (mIA1), murine P450 IA2 (mIA2), or human P450 IA2 (hIA2). Acetanilide hydroxylase (AcOH) activity was measured using an HPLC method with more than 500-fold greater sensitivity than previously described procedures. This method, which does not require the use of radioactive acetanilide, was achieved by optimizing both the gradient system and the amount of enzyme needed to achieve detection by uv light. MAb 1-7-1 inhibits up to 80% of the AcOH activity in both rat liver microsomes and cDNA expressed mouse and human P450 IA2. MAb 1-7-1, which recognizes both P450 IA1 and P450 IA2, completely inhibits the aryl hydrocarbon hydroxylase (AHH) activity of cDNA expressed in IA1. The inhibition of only 80% of the AHH activity present in MC liver microsomes by MAb 1-7-1 suggests that additional P450 forms are contributing to the overall AHH activity present in methylcholanthrene (MC)-liver microsomes as MAb 1-7-1 almost completely inhibits the AHH activity of expressed mIA1. Maximal inhibition of IA2 by 1-7-1 results in an 80% decrease in acetanilide hydroxylase activity in both liver microsomes and expressed mouse and human IA2. The capacity of MAb 1-7-1 to produce identical levels of inhibition of acetanilide hydroxylase activity in rat MC microsomes (80%) and in expressed mouse (81%) and human P450 IA2 (80%) strongly suggests that P450 IA2 is the major and perhaps the only enzyme responsible for the metabolism of acetanilide. These results demonstrate the complementary utility of monoclonal antibodies and cDNA expression for defining the contribution of specific P450 enzymes to the metabolism of a given substrate. This complementary approach allows for a more precise determination of the inhibitory capacity of MAb with respect to the metabolic capacity of the target P450.

Substrate specificities and functions of the P450 cytochromes

Currently, the major recognized biochemical functions of members of the large superfamily of P450 hemoproteins (referred to commonly as the cytochromes P450) include catalyses of the monooxygenations of a wide variety of endogenous and exogenous lipophilic chemicals. Substrates that have attracted the greatest attention thus far are steroids, fatty acids, eicosanoids, retinoids, other endogenous lipids, therapeutic agents, pesticides/herbicides, chemical carcinogens, industrial chemicals and other environmental contaminants and toxic xenobiotic organics of low molecular weight. Commonly, monooxygenation of such substrates results in the generation of metabolites capable of producing biological effects that are profoundly different (qualitatively as well as quantitatively) from those elicitable by the parent chemical per se. P45OXIX-dependent conversion of testosterone to estradiol-17 beta provides a dramatic example. Thus, these hemoproteins serve as extremely important but, as yet, largely unpredictable regulators of the biological effects producible by endobiotics as well as by xenobiotics. Current focus is on the identification and acquisition of sequence information on hereto unidentified and/or uncharacterized P450 isoforms and ascertainment of the specific functions of specific, individual isoforms. The regulation of quantities and activities of such isoforms in specific species/tissues, understandably, is also of great current interest. This interest has been further intensified by recent results indicating that substrate specificity associated with one P450 may not be the same as the corresponding isoform derived from a different animal species. Recent technological advances promise to greatly hasten the acquisition of knowledge concerning the functions of these important hemoproteins.