Sucrase-isomaltase deficiency. Absence of an inactive enzyme variant

Deficiency of sucrase-isomaltase, an intestinal enzyme complex that is essential for digestion of nutritionally important carbohydrates, appears to be inherited as an autosomal recessive in 0.2 per cent of North Americans. The molecular basis of the deficiency has not been elucidated. To characterize the enzyme defect quantitatively, papain-solubilized intestinal biopsies were analyzed for total enzyme protein by radioimmunoassay and for enzyme activity by hydrolytic assay. Normal intestine had a close correlation between immunologically reactive enzyme and enzymic activity. In contrast, seven patients with sucrase-isomaltase deficiency were found to have complete absence of the enzyme protein by radioimmunoassay even though up to 10 times more intestinal protein was present than with normal tissue. This absence of an inactive enzyme variant can be explained by a major (no-sense) mutation of the structural gene or by a complete repression of the regulatory mechanism that controls structural gene function.