The number and the reactivity of accessible thiol groups of the Folch-Pi apoprotein and proteolipid (50% of myelin proteins) were studied, by using a specific thiol-disulphide interchange reaction, in connection with the known solubility of this protein in organic and aqueous solvents. The high reactivity of 2,2′-dipyridyl disulphide towards thiol groups leads to the titration of 4.8 mol of SH groups/mol of protein (Mr 30000) in alkaline and acidic chloroform/methanol (2:1, v/v). Unlike previous findings, this value was consistently found from batch to batch and remained stable with time. In the proteolipid 1 mol of SH groups/mol was not accessible as compared with the apoprotein. In aqueous solvents, a similar number of 4.4 mol of SH groups/mol was also found. For the first time, kinetic studies carried out in chloroform/methanol discriminated between two classes of thiol groups. The reaction of 2 mol of SH groups/mol was characterized by apparent second-order rate constants whose values were 5-10-fold higher than those of the other class. Kinetic studies and cyanylation experiments in aqueous solvents also indicated the high reactivity of these thiol groups with Ellman's reagent. Together with kinetic results, studies on the stoichiometry of the interchange reaction of equimolar solutions of protein and disulphide indicate that these highly reactive thiol groups are near to each other in the amino acid sequence. The location of the thiol groups at the boundary between hydrophilic and hydrophobic domains of the Folch-Pi protein is suggested in connection with their possible structural and biological significance.

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