Immunochemical studies on the contribution of NADPH cytochrome P-450 reductase to the cytochrome P-450-dependent metabolism of arachidonic acid

We have studied the role of NADPH cytochrome P-450 reductase in the metabolism of arachidonic acid and in two other monooxygenase systems: aryl hydrocarbon hydroxylase and 7-ethoxyresorufin-o-deethylase. Human liver NADPH cytochrome P-450 reductase was purified to homogeneity as evidenced by its migration as a single band on SDS gel electrophoresis, having a molecular weight of 71,000 Da. Rabbits were immunized with the purified enzyme and the resulting antibodies were used to evaluate the involvement of the reductase in cytochrome P-450-dependent arachidonic acid metabolism by bovine corneal epithelial and rabbit renal cortical microsomes. A highly sensitive immunoblotting method was used to identify the presence of NADPH cytochrome P-450 reductase in both tissues. We used these antibodies to demonstrate for the first time the presence of cytochrome c reductase in the cornea. Anti-NADPH cytochrome P-450 reductase IgG, but not anti-heme oxygenase IgG, inhibited the NADPH-dependent arachidonic acid metabolism in both renal and corneal microsomes. The inhibition was dependent on the ratio of IgG to microsomal protein where 50% inhibition of arachidonic acid conversion by cortical microsomes was achieved with a ratio of 1:1. A higher concentration of IgG was needed to achieve the same degree of inhibition in the corneal microsomes. The antibody also inhibited rabbit renal cortical 7-ethoxyresorufin-o-deethylase activity, a cytochrome P-450-dependent enzyme. However, the anti-NADPH cytochrome P-450 reductase IgG was much less effective in inhibiting rabbit cortical aryl hydrocarbon hydroxylase. Thus, the degree of inhibition of monooxygenases by anti-NADPH cytochrome P-450 reductase IgG is variable. However, with respect to arachidonic acid, NADPH cytochrome P-450 reductase appears to be an integral component for the electron transfer to cytochrome P-450 in the oxidation of arachidonic acid.     

Carbohydrates on human Fc gamma receptors. Interdependence of the classical IgG and nonclassical lectin-binding sites on human Fc gamma RIII expressed on neutrophils

To explore the molecular basis for the ability of aggregated IgG to block the phagocytosis by human polymorphonuclear leukocytes of Con A-opsonized E and of nonopsonized Escherichia coli with mannose-binding adhesins, we examined specific aspects of the glycoprotein structure of both the 40- to 43-kDa receptor for the Fc portion of IgG (Fc gamma RII) and the 50- to 78-kDa receptor for the Fc portion of IgG (Fc gamma RIIIPMN) from human polymorphonuclear leukocytes. Fc gamma RIIIPMN isolated by both mAb and ligand affinity chromatography, but not Fc gamma RII, binds Con A in Western blots. This binding is specifically inhibitable by alpha-methylmannoside. Digestion of Fc gamma RIIIPMN by recombinant endoglycosidase H, which is specific for high mannose-type (Con A-binding) oligosaccharides, alters the epitope recognized by mAb 3G8 in or near the IgG ligand-binding site of the receptor. Similarly, the ability of Fc gamma RIIIPMN to bind human IgG ligand is sensitive to endoglycosidase H digestion. Our data indicate that ligands other than the classical IgG opsonins can bind to human Fc gamma RIIIPMN per se through lectin-carbohydrate interactions. Furthermore, Fc gamma RIIIPMN contains a high mannose type oligosaccharide chain which contributes importantly to the integrity of the classical IgG ligand-binding site. Thus, specific glycosylations of the receptor are important for both classical and nonclassical engagement of Fc gamma RIII and may play a role in determining the properties of the ligand-binding site.     

Metabolism of 20-hydroxyeicosatetraenoic acid by cyclooxygenase. Formation and identification of novel endothelium-dependent vasoconstrictor metabolites

We recently demonstrated that 20-hydroxyeicosatetraenoic acid (20-HETE) constricts rat aortic rings. The contractile response was partially dependent on the presence of endothelium and was abolished by pretreatment of the rings with either indomethacin or the endoperoxide/thromboxane receptor antagonist, SQ29548. Addition of GSH or SnCl2 to the organ bath diminished the contractile response of 20-HETE, whereas preincubation of the rings with a thromboxane synthase inhibitor did not affect the 20-HETE induced contractions. Short time incubation (2 min) of 20-HETE with ram seminal vesicle microsomes in the presence of p-hydroxymercurybenzoate yielded metabolites which migrated similarly on thin layer chromatography to the known arachidonate endoperoxides prostaglandin (PG) G2 and PGH2 and possess vasoconstrictory properties. The vasoconstriction was dose-dependent with a half-life of approximately 6.3 +/- 0.6 min. Addition of SQ29548 to the aortic ring bath 1 min after metabolite elicited vasoconstriction produced immediate relaxation. Furthermore, pretreatment of the rings with SQ29548 totally abolished the contraction. SnCl2 reduction of the metabolites produced in incubation of rat seminal vesicles with 20-HETE and p-hydroxymercurybenzoate resulted in a single radioactive peak which was further identified by gas chromatography/mass spectrometry as 20-hydroxy-PGF2 alpha. The inhibitory effect of SQ29548, the appearance of labile metabolites with a half-life of approximately 6 min and the production of 20-hydroxy-PGF2 alpha by SnCl2 reduction clearly indicate that the vasoconstrictor metabolites of 20-HETE are the labile endoperoxides of 20-HETE, 20-hydroxy-PGG2, and 20-hydroxy-PGH2.     

Biochemical actions of vasoactive peptide hormones. Time-synchronized activation of lipolysis and decreased fatty-acid release by bradykinin and angiotensin in the perfused rabbit kidney.

Bradykinin and angiotensin administered to the isolated perfused rabbit kidney activate two sequential processes: (1) a selective release of the prostaglandin precursor arachidonate with concomitant partial conversion of the arachidonate into prostaglandin E2; (2) activation of a process that leads to decreased release of all fatty acids in the perfusate. There is a time lag of approx. 1 min between the initial activation of the arachidonate-specific deacylation reaction that is coupled to prostaglandin generation, and the subsequent decrease in the release of all fatty acids. This synchronized cycle provides for instant generation of required amounts of prostaglandins and at the same time serves to conserve cellular arachidonate.     

Toxoplasma gondii: Purine synthesis and salvage in mutant host cells and parasites

Toxoplasma gondii, growing exponentially in heavily infected mutant Chinese hamster ovary cells that had a defined defect in purine biosynthesis, did not incorporate [U-¹⁴C]glucose or [¹⁴C]formate into the guanine or adenine of nucleic acids. Intracellular parasites therefore must be incapable of synthesizing purines and depend on their host cells for them. Extracellular parasites, which are capable of limited DNA and RNA synthesis, efficiently incorporated adenosine nucleotides, adenosine, inosine, and hypoxanthine into their nucleic acids; adenosine 5′-monophosphate was the best utilized precursor. Extracellular parasites incubated with ATP labeled with ³H in the purine base and ³²P in the α-phosphate incorporated the purine ring 50-fold more efficiently than they did the α-phosphate. Thus, ATP is largely degraded to adenosine before it can be used by T. gondii for nucleic acid synthesis. Two pathways for the conversion of adenosine to nucleotides appear to exist, one involving adenosine kinase, the other hypoxanthine—guanine phosphoribosyl transferase. In adenosine kinase-less mutant parasites, the efficiency of incorporation of ATP or adenosine was reduced by 75%, which indicates the adenosine kinase pathway was predominant. Extracellular parasites incorporated ATP into both the adenine and the guanine of their nucleic acids, so ATP from the host cell could supply the entire purine requirement of T. gondii. However, ATP generated by oxidative phosphorylation in the host cell is not essential for parasites because they grew normally in a cell mutant that was deficient in aerobic respiration and almost completely dependent upon glycolysis.     

Development of cell surface saccharides on embryonic pancreatic cells

Using a battery of seven lectin-ferritin conjugates as probes for cell surface glycoconjugates, we have studied the pattern of plasmalemmal differentiation of cells in the embryonic rat pancreas from day 15 in utero to the early postpartum stage. Our results indicate that differentiation of plasmalemmal glycoconjugates on acinar, endocrine, and centroacinar cells is temporally correlated with development and is unique for each cell type, as indicated by lectin-ferritin binding. Specifically, (a) expression of adult cell surface saccharide phenotype can be detected on presumptive acinar cells as early as 15 d in utero, as indicated by soybean agglutinin binding, and precedes development of intracellular organelles characteristic of mature acinar cells; (b) maturation of the plasmalemma of acinar cells is reached after intracellular cytodifferentiation is completed, as indicated by appearance of Con A and fucoselectin binding sites only at day 19 of development; conversely, maturation of the endocrine cell plasmalemma is accompanied by "loss" (masking) of ricinus communis II agglutinin receptors; and (c) binding sites for fucose lectins and for soybean agglutinin are absent on endocrine and centroacinar cells at all stages examined. We conclude that acinar, centroacinar, and endocrine cells develop from a common progenitor cell(s) whose plasmalemmal carbohydrate composition resembles most closely that of the adult centroacinar cell. Finally, appearance of acinar lumina beginning at approximately 17 d in utero is accompanied by differenetiation of apical and basolateral plasmalemmal domains of epithelial cells, as indicated by enhanced binding of several lectin-ferritin conjugates to the apical plasmalemmal, a pattern that persists from this stage through adult life.