Accessibility of glucose 6-phosphate: phosphohydrolase to antibody attack in modified microsomal vesicles.

Recent investigations supported the existence of 2 components of the endoplasmic reticulum participating in the process of glucose 6-phosphate hydrolysis: the glucose 6-phosphate-specific transporter that mediates the movement of the substrate from the cytoplasmic membrane surface into the lumen and the unspecific phosphohydrolase on the luminal side of the membrane [ 1,2]. However, this model has not been generally accepted; especially the proposed molecular arrangement of the glucose 6-phosphatase components within the membrane is contradictory [3-71. Furthermore, immunological studies [6] have suggested that the glucose 6-phosphate: phosphohydrolase is presumably not freely accessible on the luminal surface which, however, is one of the prerequisites of the substrate-transport hypothesis as described in [ 1,2]. Therefore, we have reinvestigated the transverse topology of the glucose 6-phosphatase by detailed immunological studies on detergent-modified and mechanically disrupted microsomes. These findings support our preliminary concept and demonstrate that, indeed, the glucose 6-phosphate:phosphohydrolase is not attached to the luminal membrane surface, but buried within the microsomal membrane.