THE MOLECULAR-BASIS OF THE GENETIC DEFICIENCIES OF 5 OF THE COMPONENTS OF THE GLUCOSE-6-PHOSPHATASE SYSTEM - IMPROVED DIAGNOSIS

The understanding of type 1 glycogen storage diseases (GSDs) has been greatly hindered by a lack of knowledge of the molecular basis of gluc ose-6-phosphatase (Glc-6-P'ase). The problem has been the complete fai lure of many laboratories, including our own, to purify to homogeneity a single polypeptide with high levels of Glc-6-P'ase activity. The be st preparations to date all contain five or six different polypeptide bands and have specific activities in the range 17-50 mumoles/min per milligram. The two major reasons for failure have been that Glc-6-P'as e is extremely difficult to solubilise from the microsomal membrane (l arge amounts of detergents are needed) and that it is not a single pol ypeptide as originally thought, but a multicomponent system. Recent st udies of patients with type 1 GSD have proved that Glc-6-P'ase compris es at least five different polypepetides. Four of the proteins have no w been purified and three have been cloned. We have assayed the Glc-6- P'ase system in over 600 human biopsy samples and developed microassay s to diagnose deficiencies of each of the proteins. Ways of avoiding p ossible problems which have the potential to lead to the wrong diagnos is will be discussed.