Cellular and Subcellular Localization of Hexokinase, Glutamate Dehydrogenase, and Alanine Aminotransferase in the Honeybee Drone Retina

Abstract

Subcellular localization of hexokinase in the honeybee drone retina was examined following fractionation of cell homogenate using differential centrifugation. Nearly all hexokinase activity was found in the cytosolic fraction, following a similar distribution as the cytosolic enzymatic marker, phosphoglycerate kinase. The distribution of enzymatic markers of mitochondria (succinate dehydrogenase, rotenone‐insensitive cytochrome c reductase, and adenylate kinase) indicated that the outer mitochondrial membrane was partly damaged, but their distributions were different from that of hexokinase. The activity of hexokinase in purified suspensions of cells was fivefold higher in glial cells than in photoreceptors. This result is consistent with the hypothesis based on quantitative 2‐deoxy[3H]glucose autoradiography that only glial cells phosphorylate significant amounts of glucose to glucose‐6‐phosphate. The activities of alanine aminotransferase and to a lesser extent of glutamate dehydrogenase were higher in the cytosolic than in the mitochondrial fraction. This important cytosolic activity of glutamate dehydrogenase was consistent with the higher activity found in mitochondria‐poor glial cells. In conclusion, this distribution of enzymes is consistent with the model of metabolic interactions between glial and photoreceptor cells in the intact bee retina.

A glia–neuron alanine/ammonium shuttle is central to energy metabolism in bee retina

“…Glucose did not restore in the presence of DAB. This confirms that glucose itself cannot be the substrate supplied by the glia, as predicted from the glial‐specific uptake of 2‐deoxyglucose (Tsacopoulos et al 1988), and the presence of only a weak hexokinase activity in isolated photoreceptors (Veuthey et al 1994). Unexpectedly, DAB also prevented exogenous glucose from being used by the glia to produce alanine, or another substrate (Figs 4, 7).…”

Insect models of central nervous system energy metabolism and its links to behavior

“…Here, an extreme form of metabolic division of labor between the glia and the photoreceptor neurons occurs. A key rate‐limiting glycolytic enzyme, hexokinase, is found predominantly in the glial cells (Veuthey, Tsacopoulos, Ruiz, & Perrottet, ). In accordance with this, retinal slices bathed in 2‐deoxyglucose (2‐DG, a glucose analog that is taken up into cells and phosphorylated, but then cannot be processed further or released), showed accumulation of 2‐DG exclusively in glial cells and not in the photoreceptors, indicating that glycolysis is only found in the glia (Tsacopoulos et al, ).…”