GLUCOSE-6-PHOSPHATASE IN THE SALIVARY GLANDS OF SCIARA COPROPHILA: A HISTOCHEMICAL AND BIOCHEMICAL STUDY

Biochemical characteristics of an enzyme in larval stages of Sciara coprophila specifically hydrolyzing glucose-6-phosphate are shown to parallel those of a similar enzyme in mammalian liver with respect to substrate specificity and reactivity in the presence of various inhibitors. Even with prolonged incubation this enzyme can be demonstrated histochemically in unfixed larval salivary gland of Sciara coprophila only in focal dilated cisternae of the rough surfaced endoplasmic reticulum. It was concluded that the morphologically homogeneous microsomal system may have areas of biochemical specialization which allow the production of specific proteins (in this case a glucose-6-phosphatase) made in no other portion of the rough surfaced endoplasmic reticulum. Glucose-6-phosphatase activity is absent biochemically during the 2nd and 3rd instars. A minimal amount is present in the 4th instar and a marked elevation occurs prior to pupation; this level is maintained during pupation. Histochemically, glucose-6-phosphatase activity can be demonstrated throughout the larval stage. The relationship of the characteristic 4th instar puffs of dipteran larval chromosomes to the onset of high levels of glucose-6-phosphatase activity is discussed.