N-Methylprotoporphyrin dimethyl ester inhibits ferrochelatase in isolated membranes of Rhodopseudomonas sphaeroides at low concentrations (around 10 nm). Full inhibition developed after a short lag phase. The inhibition was non-competitive with porphyrin substrate. Addition of inhibitor to growing cultures of Rps. sphaeroides caused a decrease (near 40%) in cytochrome content and a severe inhibition of ferrochelatase; the excretion of haem into the medium by cell suspensions was also severely inhibited. The addition of N-methylprotoporphyrin dimethyl ester to suspensions of photosynthetically competent Rps. sphaeroides Ga caused excretion of Mg-protoporphyrin monomethyl ester. When added to mutants V3 and O1, magnesium divinylphaeoporphyrin a5 monomethyl ester and 2-devinyl-2-hydroxyethylphaeophorbide a were excreted, with maximum effect at around 3 microM-inhibitor in the medium. The results are interpreted to suggest that the inhibitor decreases concentration of intracellular haem, which normally controls the activity of 5-aminolaevulinate synthetase. Unregulated activity of this enzyme leads to overproduction of protoporphyrin, which is diverted to the bacteriochlorophyll pathway. Further control operates at magnesium protoporphyrin ester conversion in normal cells.
The interplaying mechanisms involved in the electrocatalytic oxidation of hydrogen peroxide (H202) have been studied using cyclic and differential-pulse voltammetry and X-ray photoelectron spectroscopy (XPS). Cobalt phthalocyanine (CoPC) modified screen-printed carbon electrodes (SPCEs) were used as base transducers in the voltammetric studies, and for XPS experiments a mixture of the printing ink containing CoPC was coated onto a steel stub. These investigations have shown that CoPC is initially in the Co2+ oxidation state, and that in plain supporting electrolytes it is subsequently electrochemically oxidized to Co3+. A postulated sequence of events for the mediated oxidation of H202 at CoPC-SPCEs has been derived from the XPS and voltammetric data generated.
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