Effect of lipids and organic solvents on the enzymic formation of zinc protoporphyrin and haem

Abstract

Differences observed in earlier work between the enzymic chelation with protoporphyrin of Zn2+ and Fe2+ ions respectively have now been explained as being caused by the presence of peroxides in the ether used in the enzyme assay. The inhibitory effect of peroxides is established by the reducing agent, which is present in the assay for chelation of Fe but not in that for Zn. There are now no reasons for the belief that 2 different enzymes catalyze formation of complexes with Zn and Fe respectively. Removal of lipid from both chromatophores and mitochondria markedly reduced chelatase activity. Activity could be partially restored by the addition of lipid fractions. Phos-phatidic acid, but not phosphatidylcholine or phosphatidylethanolamine, actively stimulated the formation of Zn protoporphyrin and heme by chromatophores and mitochondrial preparations. Lipid-containing extracts of chromatophores, and fractions thereof obtained by silicic acid chromatography, were active in restoring chelatase activity of Tween extracts of mitochondria. Thus, although both enzymes are considered to be lipoproteins, the identity of the lipids concerned is still uncertain. A great number of organic solvents such as esters, ethers, ketones and, to a lesser extent, alcohols, stimulate enzymic chelation of both metals with protoporphyrin. A number of explanations for these findings are considered and it is suggested that organic solvents interact in some way with the enzyme lipoprotein, changing either its conformation or allowing closer contact between the enzyme and its substrates.