The number of total disulfide bonds in rabbit and sheep 7S gamma globulin, before and after treatment with 2-mercaptoethanol, has been measured by amperometric titration.

Mercaptan reduction could diminish the complement-fixing efficiency of 7S rabbit gamma globulin by no more than 90 per cent without any significant decrease in maximal complement-fixing ability. This was associated with the reduction of only 7 disulfide bonds, including 1 interchain disulfide bond [S—S(1)].

The reduction of 7 disulfide bonds in 7S sheep gamma globulin was associated with a 64 per cent decrease in complement-fixing efficiency, while reduction of 10 to 11 disulfide bonds decreased the complement-fixing efficiency by 90 per cent. Reduction of more disulfide bonds was not associated with any further decrease in complement-fixing efficiency, nor of any decrease in maximal complement-fixing ability.

The reduction of more than 10 to 11 disulfide bonds in 7S rabbit and sheep gamma globulins was associated with a decrease in precipitating ability.

The disulfide bonds which are more labile to mercaptan reduction appear to be associated with complement-fixing efficiency while the disulfide bonds which are more resistant to mercaptan reduction appear to be associated with precipitating ability.

The measurement of interpolypeptide chain disulfide bonds has been discussed. One easily reduced interchain disulfide bond [S—S(1)] appears to be important for the complement-fixing efficiency of 7S rabbit antibody. The integrity of this same bond is essential for the precipitating ability of 5S rabbit antibody and may also be important for its complement-fixing ability.

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