Enhancement of microsomal aniline and acetanilide hydroxylation by haemoglobin

Hydroxylation and dealkylation reactions catalyzed by hemoglobin

Red blood cells contain many enzymes that are akin to those that catalyze xenobiotic metabolism in liver and other tissues. An obvious exception is the cytochrome P-450 system that is found in virtually all other tissues. In vitro studies, however, have shown that hemoglobin can be a broad monooxygenase catalyst, exhibiting the properties of a monooxygenase enzyme. Thus, catalysis by Hb displays typical Michaelis-Menten kinetics, dependence on the native protein, coupling to NADPH-dependent flavoprotein reductases, and inhibition by carbon monoxide. The reconstituted system containing Hb along with P-450 reductase utilizes NADPH and O2 to catalyze typical monooxygenase reactions, including O- and N-demethylations as well as aromatic and aliphatic hydroxylations, and the catalytic cycle appears to mimic the typical P-450 mechanism. Turnover numbers for aniline hydroxylation are similar for Hb and P-450 reconstituted systems, whereas P-450 systems are more effective for other reactions. Catalysis by Hb seems to be restricted to the beta-heme sites of the tetramer, reflecting more facile substrate access. Overall the similarities and differences between Hb and P-450 provide an opportunity to examine the basis for their differential monooxygenase or peroxidase/peroxygenase activities in a comparative manner. Hb may be especially useful in delineating the early events in the respective reaction schemes, because it can be studied in various stable redox/ligand states, including the oxyferrous form. Similar hemoglobin-catalyzed oxidative biotransformations occur within intact erythrocytes, but apparent turnover numbers are much lower than those with the reconstituted Hb system, suggesting different mechanisms of catalysis. Although Hb-mediated oxidase activity in erythrocytes is low relative to other sites of xenobiotic metabolism, it may contribute to in situ activation of xenobiotics leading to oxidative stress, disruption of sulfhydryl homeostasis in the erythrocytes, covalent modification of Hb, and hemolysis.

Interactions with hemoglobin: a source of error in measurements of transketolase activity in hemolysates

Measurements of the activity of transketolase in human erythrocyte lysates by an assay coupled to NADH oxidation indicate that interactions of assay substrates with hemoglobin can give rise to overestimations of transketolase activity. Three potential sources of error are identified. Thus, in lysates containing methemoglobin, NADH oxidation can be due firstly to methemoglobin reductase activity or secondly to the monooxygenase activity of methemoglobin, for which the substrate can be ribose 5-phosphate, a substrate also of transketolase. Thirdly, the addition of high concentrations of the transketolase cofactor, TDP, to an insufficiently buffered reaction mixture can cause the aggregation and precipitation of hemoglobin: a phenomenon that may be misconstrued as an enhanced increase in absorbance at 340 nm and hence as additional transketolase activity. Although the present study concentrates on these potential artefacts in assays of transketolase activity, the findings may well be relevant to the measurement of other enzyme activities in hemolysates by procedures based ultimately on the rate of consumption or production of NAD(P)H.

Monooxygenase activity of human hemoglobin: role of quaternary structure in the preponderant activity of the beta subunits within the tetramer

Catalysis of para hydroxylation of aniline was measured for human ferrihemoglobin and various derivatives in a reconstituted system consisting of the appropriate hemoprotein (at 4 microM heme), reduced nicotinamide adenine dinucleotide phosphate (NADPH), cytochrome P-450 reductase, and aniline under atmospheric O2. The isolated subunits of hemoglobin (alpha 3+ and beta 3+4) were prepared by treatment with p-(hydroxymercuri)benzoate. Semihemoglobin (alpha heme2 beta 02) was prepared from ferrihemoglobin and apohemoglobin. Converse valency hybrids alpha 3+2(beta 2+-CO)2 and (alpha 2+-CO)2 beta 3+2 were prepared from appropriately ligated alpha and beta subunits. After chromatography, the hemoglobin derivatives were characterized by visible and 1H NMR spectroscopy and electrophoresis. At the same concentration of aniline, the alpha and beta subunits were much less active than the normal tetramer. alpha-Semihemoglobin and the alpha 3+2(beta 2+-CO)2 hybrid also displayed lower hydroxylase activity. The (alpha 2+-CO)2 beta 3+2 hybrid was about as active as normal alpha 3+2 beta 3+2. This result suggests that the activity of tetrameric hemoglobin primarily involves the beta subunits. Also transfer of the beta subunits from the beta 4 molecular environment to the alpha 2 beta 2 state enhances their monooxygenase activity approximately 15-fold. The hemoglobin derivatives were differently susceptible to substrate inhibition, the beta 4 species being most sensitive. Estimates of Vmax from the linear portions of the corresponding Lineweaver-Burk plots showed agreement within a factor of 2.5 for all of the hemoglobin derivatives, suggesting that the intrinsic O2-activating capacities of the derivatives are similar.(ABSTRACT TRUNCATED AT 250 WORDS)

Biotransformation of nitrosobenzene, phenylhydroxylamine, and aniline in the isolated perfused rat liver

1. Haemoglobin-free single-pass perfusion of isolated rat liver with [14C]aniline, [14C]phenylhydroxylamine, and [14C]nitrosobenzene was carried out. 2. Perfusion with aniline revealed apparent enzyme kinetics for 4-aminophenol formation with Km = 144 microM, Vmax = 51 nmol/min per g liver wet; for 2-aminophenol Km = 144 microM, Vmax = 16 nmol/min per g; for acetanilide Km = 33 microM, Vmax = 25 nmol/min per g. Formation of phenylhydroxylamine and nitrosobenzene was observed at a rate of 1.5 nmol/min per g provided that these metabolites had been trapped within red cells. 3. Perfusion with phenylhydroxylamine displayed a metabolic pattern similar to aniline with apparent phenylhydroxylamine reduction kinetics of Km = 260 microM and Vmax = 600 nmol/min per g. In addition an acid-labile phenylhydroxylamine glucuronide was formed. 4. Perfusion with nitrosobenzene showed very rapid reduction to phenylhydroxylamine and to the metabolites observed with phenylhydroxylamine. In postmicrosomal supernatant, enzymic reduction of nitrobenzene by NADH and NADPH showed Km = 12 microM nitrosobenzene and Vmax = 5000 nmol/min per g. 5. Three per cent of nitrosobenzene was irreversibly bound to liver proteins. After 20 min perfusion with nitrosobenzene, 0.95 mumol of liver glutathione was lost per 10 mumol nitrosobenzene infused; 0.16 mumol of glutathione was released with effusate and bile, 0.46 mumol of glutathionesulphinanilide was produced, the rest, 0.33 mumol, may have formed mixed disulphides.

Enhancement of nitro reduction in rat liver microsomes by haemin and haemoproteins

1. Reductive metabolism of p-nitrobenzoic acid and neoprontosil in rat liver microsomes was studied in the presence of haemin, haemoglobin and myoglobin. 2. Microsomal nitro reduction is enhanced 4-fold in the presence of haemoglobin, whereas azo reduction is not affected. 3. Microsomal nitro reduction is enhanced to a similar extent by haemoglobin, haemin and boiled haemoglobin, whereas myoglobin is about half as active. 4. Maximal enhancement of microsomal nitro reductase activity by haemoglobin is achieved at high substrate concentration (6 mM) and low microsomal protein concentration (0.5--1.0 mg/ml). 5. Control microsomal nitro reduction as well as the haemoglobin-enhanced microsomal nitro reduction are inhibited completely by O2 and CO whereas potassium azide as a ligand of ferric haem iron is a less potent inhibitor.