A histochemical principle bearing on correctness of diagnosis in storage diseases

A principle is proposed which may help pathologists avoid errors in diagnosis of storage diseases. Tissues from patients in whom a tentative diagnosis of a metabolic disorder has been made often store a number of metabolites in the cells. The presence of these metabolites can occur in single-enzyme or activator defects as a result of the following causes: (a) deposition of metabolites situated near the main substrate of the defective enzyme in the catabolic path, and compounds which were changed after they were deposited; (b) presence of multiple substrates for this enzyme; (c) co-deposition of molecules bound to the main substrates; (d) existence of multiple substrates for a single defective activator molecule. In contrast to these causes, variability in processes not associated with a single-enzyme or activator deficiency may be due to the following: (e) inhibition of multiple hydrolases by drugs or metabolites; (f) localization of substrates and hydrolases in different compartments; (g) multiple enzyme deficiencies; (h) concentration of metabolites beyond the catabolic capacity of cells. According to the proposed principle, diagnosis of storage disease resulting from a single enzyme deficiency can be negated if a wide-range histochemical test shows that the main substrate of a deficient enzyme is not present in some primary storage cells. The validity of the principle and possible pitfalls are discussed.