The determination of osteopetrotic phenotypes by selective inactivation of red cell carbonic anhydrase isoenzymes

Abstract

Red cell carbonic anhydrase isoenzyme activities (HCA-I and HCA-II) were quantitated in blood hemolysates of two female siblings affected with autosomal recessive osteopetrosis in addition to family members who were both obligate and potential heterozygotes as well as normal controls. Selective inactivation of red cell hemolysates with 20 mmol/l bromopyruvic acid (pH 7.5) for 50 min at 25 degrees C lead to a near quantitative removal of HCA-I activity without significant loss of HCA-II activity. Comparison of untreated hemolysate HCA activity with residual activity in bromopyruvate treated samples allowed determination of HCA-II/HCA-I activity ratios. Estimation of activity ratios was also attempted by use of the inhibitor I- which is known to discriminate between the two isoenzymes. Hemolysate HCA activity of the two affected individuals (homozygotes) was completely abolished by the procedure, consistent with an absence of the bromopyruvate resistant HCA-II isoenzyme in the red cells of these individuals. HCA-I activities were found to be elevated significantly above normal. Both parents (obligate heterozygotes) were found to have normal HCA-I activities and HCA-II activities 60-80% of normal. Computed HCA-II/HCA-I activity ratios fell into these ranges: 0 for homozygotes, 0.82-0.87 for obligate heterozygotes and 1.08-1.16 for normal controls. KI values for I- inhibition of hemolysate HCA activity gave 0.25-0.30 mmol/l, 0.9-1.5 mmol/l and 2.0 mmol/l for the three categories. These relations should prove useful in the screening of osteopetrotic phenotypes.