Human alpha-fetoprotein and albumin: differences in free sulfhydryl groups

The number of free sulfhydryl (SH) groups of both human alpha-fetoprotein (AFP) and albumin, isolated simultaneously from human cord serum, was determined by reacting the proteins with 5,5'-dithiobis (2-nitrobenzoic acid) (DTNB). A higher number of SH groups was consistently found with AFP than with albumin. When both proteins were pretreated with 100 mmol/l 2-mercaptoethanol, only approximately 0.5 SH groups were detected per molecule of human adult albumin, yet as many as 4 per molecule were found with AFP. Dithiothreitol was found to be more effective than 2-mercaptoethanol in SH group regeneration. The results suggested that some disulfide linkages of AFP could be reduced. Free SH groups regenerated by the treatment with SH reagent were found to disappear gradually during storage under aerobic conditions. The rate of disappearance differed between AFP and albumin.