Type Ib Glycogen Storage Disease: An In Vivo and In Vitro Study of Two Cases

Type Ib glycogen storage disease (type Ib GSD) is characterized by clinical and biochemical features of glucose-6-phosphatase deficiency (type Ia GSD). However, the activity of the glucose-6-phosphatase (G-6-Pase, EC 3.1.3.9) is normal in frozen liver specimens and markedly low in fresh tissue. It was postulated by Narisawa et al. (1978) that type 1b GSD is caused by a defect in microsomal glucose-6-phosphate (G-6-P) transport leading to a non-functional G-6-Pase in vivo. In connection with this, Van Hoof et al. (1972) proposed a double isotopic glucose test for the evaluation of the enzyme in vivo. In this paper, we report in vitro and in vivo assays of G-6-Pase in two patients with severe clinical symptoms.