Solubilization characteristics of pig liver S-methyltransferase

S-methyltransferase was solubilized from pig liver microsomes by five different types of detergents: Triton X-100, zetyltrimethyl-ammonium bromide, sodium cholate, Zwittergent, and sodium dodecylsulfate. As regards enzyme activity, stability, and critical detergent concentration, Zwittergent proved superior to the four other detergents utilized. A striking difference was found in the catalytic activity relative to the chain length of homologous substrates between the microsomal and the solubilized enzyme. The microsomal enzyme preparation showed highest activity towards C1 substrate (methane thiol) while the solubilized enzyme had its maximal activity for C7 substrate. This observation accorded with the active site affinity for the corresponding substrates. From the reaction of the enzyme to the different detergents and the distribution coefficient of substrates in membranes and cytosol, it can be deduced that the enzyme is located on the surface of microsomal membranes with the active site directed towards the cytoplasm.