Isolation and functional characterization of a cyst(e)amine-sensitive protein protecting methyl-dithio groups

We have previously shown that S-methylthio cysteine mixed disulfide and cystamine potently stimulate thiol production and glutathione synthesis of a human T-cell line in SH-poor medium. Here, we describe a simple photometric method for the determination of methylthio-mixed disulfides (MT-groups) and show that liver contains relatively large amount of MT-groups mainly associated with the globulin fractions. At least a part of methylthio (MT-) globulins is in a complex with a heat-stable protein protecting methylthio-groups against reduction and was designated as anti-methylthio factor (AMTF). Similar complexes are present in some animal sera. AMTF isolated from mouse liver was shown to specifically inhibit redox interaction of methyldithio-groups of various origin with thiols but loses this ability in the presence of some agents such as cyst(e)amine, 2-mercaptoethanol and dimethyl sulfoxide abrogating the MT-binding activity of AMTF (= cystamine-sensitive protein-CSP). AMTF purified by heat treatment and isopropanol fractionation was chromatographed on Superose-12 column. Preliminary results showed that the molecular mass of the active component is about 34 kDa consisting of two identical subunits. The possible biological role of MT-AMT complexes was discussed.