Identification of protein components of the microsomal glucose 6-phosphatetransporter by photoaffinity labelling

The glucose-6-phosphatase system catalyses the terminal step of hepatic glu cose production from both gluconeogenesis and glycogenolysis and is thus a key regulatory factor of blood glucose homoeostasis. To identify the glucos e 6-phosphate transporter TI, we have performed photoaffinity labelling of human and rat liver microsomes by using the specific photoreactive glucose- 6-phosphate translocase inhibitors S 0957 and S 1743. Membrane proteins of molecular mass 70, 55, 33 and 31 kDa were labelled in human microsomes by [ H-3]S 0957, whereas in rat liver microsomes bands at 95, 70, 57, 54, 50, 41 , 33 and 31 kDa were detectable. The photoprobe [H-3]S 1743 led to the pred ominant labelling of a 57 kDa and a 50 kDa protein in the rat. Stripping of microsomes with 0.3 % CHAPS retains the specific binding of Tl inhibitors; photoaffinity labelling of such CHAPS treated microsomes resulted in the l abelling of membrane proteins of molecular mass 55, 33 and 31 kDa in human liver and 50, 33 and 31 kDa in rat liver. Photoaffinity labelling of human liver tissue samples from a healthy individual and from liver samples of pa tients with a diagnosed glycogen-storage disease type Ib (GSD type Ib; von Gierke's disease) revealed the absence of the 55 kDa protein from one of th e patients with GSD type 1. These findings support the identity of the gluc ose 6-phosphate transporter T1, with endoplasmic reticulum protein of molec ular mass 50 kDa in rat liver and 55 kDa in human liver.