Aniline hydroxylase activities of haemoglobin: Kinetics and mechanism

Catalase-like activity of bovine met-hemoglobin: interaction with the pseudo-catalytic peroxidation of anthracene traces in aqueous medium

Hemoglobin is a member of the hemoprotein superfamily whose main role is to transport O(2) in vertebrate organisms. It has two known promiscuous enzymatic activities, peroxidase and oxygenase. Here we show for the first time that bovine hemoglobin also presents a catalase-like activity characterized by a V(max )of 344 microM/min, a K(M )of 24 mM and a k(cat) equal to 115/min. For high anthracene and hemoglobin concentrations and low hydrogen peroxide concentrations, this activity inhibits the expected oxidation of anthracene, which occurs through a peroxidase-like mechanism. Anthracene belongs to the polycyclic aromatic hydrocarbon (PAH) family whose members are carcinogenic and persistent pollutants found in industrial waste waters. Our results show that anthracene oxidation by hemoglobin and hydrogen peroxide follows a typical bi-bi ping-pong mechanism with a V(max) equal to 0.250 microM/min, K(M(H2O2) )of 80 microM, K(M(ANT)) of 1.1 microM and k(cat) of 0.17/min. The oxidation of anthracene is shown to be pseudo-catalytic because an excess of hemoglobin and hydrogen peroxide is required to make PAH completely disappear. Thus, bovine hemoglobin presents, in different degrees, all the catalytic activities of the hemoprotein group, which makes it a very interesting protein for biotechnological processes and one with which structure-activity relationships can be studied.

Study of an enzyme coupled system for the development of fibre optical bilirubin sensors

We have established an analytical procedure for bilirubin (BR) determination by following the variation of its yellow absorption band in the presence of the biocatalytic system haemoglobin/glucose oxidase/glucose. The influence of the pH of the medium, the haemoglobin/hydrogen peroxide ratio, and the concentration of the various components in the mixture on the system activity was evaluated. The solubility of the unconjugated BR was analysed. An anionic detergent in Tris buffer, in which it presents a high critical micellar concentration, was selected for the BR/bovine serum albumin cleavage. The Michaelis-Menten constant of the system (after the optimisation of the various parameters) was determined and compared with Km values reported in the literature for similar catalytic systems. Preliminary results obtained with the haemoglobin and glucose oxidase immobilised either in poly(vinyl alcohol) or by crosslinking with glutaraldehyde, show that this system is promising as a future biosensor.

Is hemoglobin a catalyst for sulfoxidation of chlorpromazine? An investigation with isolated purified hemoglobin and hemoglobin in monooxygenase and peroxidase mimicking systems

The possible role of hemoglobin in the sulfoxidation of chlorpromazine is still a controversial subject. Therefore this sulfoxidation was investigated with purified oxyhemoglobin and methemoglobin under various conditions: (i) in phosphate buffer pH 6.5; (ii) in monooxygenase mimicking systems with electron donors like ascorbic acid and NADPH, the last, with and without an electron carrier like methylene blue and cytochrome c reductase; (iii) in the presence of H2O2. Only in the presence of H2O2 chlorpromazine was converted into chlorpromazine sulfoxide in a considerable amount. This so-called peroxidase activity of hemoglobin appeared not to be based on a Fenton-type reaction. An oxidized reactive form of hemoglobin (i.e. ferrylhemoglobin) is responsible for the sulfoxidation. In the other systems only with ascorbic acid some chlorpromazine sulfoxide was produced. This is probably due to the production of H2O2 and the subsequent peroxidase activity of hemoglobin. Chlorpromazine enhanced the autoxidation of oxyhemoglobin, without being transformed itself.

Oxene transfer, electron abstraction, and cooxidation in the epoxidation of stilbene and 7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene by hemoglobin

Hemoglobin plus H2O2 oxidizes trans-stilbene to trans-stilbene oxide, cis-stilbene to cis- and trans-stilbene oxide, and trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene to anti-trans-7,8,9,10-tetrahydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene. Oxidation of cis- and trans-stilbene to the corresponding cis- and trans-epoxides proceeds exclusively with incorporation of oxygen from the peroxide. Oxidation of cis-stilbene to the trans-epoxide, however, proceeds without detectable incorporation of oxygen from the peroxide and partial incorporation of oxygen from O2. The epoxidations in which stereochemistry is conserved thus appear to involve ferryl oxygen transfer, whereas the epoxidations in which stereochemistry is inverted are proposed to involve protein-mediated cooxidation [Ortiz de Montellano, P.R., & Catalano, C.E. (1985) J. Biol. Chem. 260, 9265-9271] and possibly electron abstraction-water addition. The epoxidation of trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene incorporates oxygen from H2O2 and H2O but not O2. The oxidation of this substrate is thus consistent with ferryl oxygen transfer and electron abstraction but not protein-mediated cooxidation.

Aromatic hydroxylations in peroxidations by haemoglobin systems

The catalytic activity of haemoglobin on aromatic substrates was studied in three systems: NADH-methylene blue-haemoglobin, ascorbic acid-haemoglobin, and red blood cells. Aniline and phenol but not acetanilide or p-toluidine are hydroxylated by haemoglobin. Dealkylations are not observed. Hydroxylations are postulated to be intermediate reactions in peroxidations catalysed by haemoglobin. The lifetime of the products depends on the presence of electron donors, such as NADH or ascorbic acid, in the medium. In the red blood cells where endogenous electron donors are recycled, levels of the products are higher and their lifetime is longer. This could have implications on drug metabolism by haemoglobin, as haemoglobin is present in large quantities in the organism.