pH controlled diazo coupling of aldolase. Selective formation of diazothioether chromophores and retention of enzyme activity.

pH Conditions have been found which achieve selective reaction of diazotized p-amino benzoate with cysteine residues of rabbit muscle aldolase. The difference in reactivity of the two sulphydryl groups involved, (Cys--237 and Cys--287) permits one to form either four or eight diazothioethers on the tetrameric enzyme and obtain a homogeneous protein. In both cases the enzyme became slightly more active in the fructose-1, 6-bisphosphate cleavage, the KM value being retained. The results have been discussed with regard to chemically modifying an enzyme to change its physical, chemical and immunological properties, whilst leaving the catalytical activity unmodified.