Esterification of hemins with trimethyloxonium tetrafluoroborate

Enzymic conversion of alpha-oxyprotohaem IX into biliverdin IX alpha by haem oxygenase

Conversion of four isomers of meso-oxyprotohaem IX into the corresponding biliverdin IX was attempted with a reconstituted haem oxygenase system in the presence of NADPH-cytochrome c reductase and NADPH. Only the alpha-isomer of meso-oxyprotohaem IX was converted effectively into biliverdin IX alpha, which was further reduced to bilirubin IX alpha by biliverdin reductase. Only trace amounts of biliverdins IX beta, IX gamma and IX delta were respectively formed from the incubation mixture of the corresponding oxyprotohaemin IX isomers with the complete haem oxygenase system under the same conditions. In a kinetic study, the Km for alpha-meso-oxyprotohaem IX was 3.6 microM, which was 2-fold higher than that for protohaem IX. The maximum velocity (Vmax.) of the conversion of alpha-meso-oxyprotohaem IX into biliverdin IX alpha was twice as fast as that of protohaem IX. These results demonstrate that alpha-meso-oxyprotohaem IX is an intermediate of haem degradation and it was converted stereospecifically into biliverdin IX alpha via verdohaem IX alpha.

Mechanism-based inactivation of horseradish peroxidase by sodium azide. Formation of meso-azidoprotoporphyrin IX

Catalytic turnover of sodium azide by horseradish peroxidase, which produces the azidyl radical, results in inactivation of the enzyme with KI = 1.47 mM and kinact = 0.69 min-1. Inactivation of 80% of the enzyme requires approximately 60 equiv each of NaN3 and H2O2. The enzyme is completely inactivated by higher concentrations of these two agents. meso-Azidoheme as well as some residual heme are obtained when the prosthetic group of the partially inactivated enzyme is isolated and characterized. Reconstitution of horseradish peroxidase with meso-azidoheme yields an enzyme without detectable catalytic activity even though reconstitution with heme itself gives fully active enzyme. The finding that catalytically generated nitrogen radicals add to the meso carbon of heme shows that biological meso additions are not restricted to carbon radicals. The analogous addition of oxygen radicals may trigger the normal and/or pathological degradation of heme.

High-pressure liquid chromatography and field desorption mass spectrometry of heme alpha, heme alpha dimethyl ester and acetyl heme alpha dimethyl ester

Field desorption mass spectrometry was shown to be a valid technique for determining the molecular weights of hemins and hemin esters, as well as of porphyrins. The observed base peaks of ligand-free protoheme IX, protoheme IX dimethyl ester, and protoporphyrin IX dimethyl ester correspond well to the molecular weights of these compounds and the base peak for hematoporphyrin corresponds to the molecular weight of this porphyrin minus two molecules of water. The technique was employed to confirm the molecular weights of heme alpha, heme alpha dimethyl ester, and acetyl heme alpha dimethyl ester. Heme alpha dimethyl ester was prepared by reaction of heme alpha with trimethyloxonium tetrafluoroborate and purified by high-pressure liquid chromatography. The isolated product was converted to the acetylated derivative by reaction with acetic anhydride and was subsequently purified by high-pressure liquid chromatography. A field desorption spectrum of heme alpha shows a base peak at 582 which is in agreement with previous deductions of the structure of this prosthetic group. Base peaks of the heme alpha ester and its acetylated derivative demonstrate that the two carboxyl groups have been methylated and the single hydroxyl group has been acetylated without further alteration of the molecule.