Abstract
Protein carboxymethylase (EC 2.1.1.24) from cytosol of bovine brain was found to exist as two apparent isozymes that could be separated by chromatography on DEAE-cellulose at pH 8.0. Rechromatography of the two forms, designated PCM I and PCM II, indicated that they are not interconvertible. Both enzymes have a molecular weight of 24,300 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, PCM I consists mainly of one isoelectric form, pI 6.5, whereas PCM II resolves into two forms of pI 5.6 and 5.7. The relative amounts of PCM I and PCM II show a marked tissue dependence. Brain has approximately twice as much PCM I as II, whereas liver contains only the type II enzyme. The two enzymes were found to have similar substrate specificities when tested with five different methyl-accepting proteins. Synapsin I, a basic protein associated with synaptic vesicles, was found to be an excellent methyl-accepting protein with regard to its Km (1.2 microM), but it exhibited a low stoichiometry of methyl incorporation.
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