Iodoacetate inhibits the biosynthesis of alanine in glial cells and its utilization in photoreceptors of the honeybee drone (Apis mellifera) retina

Abstract

The aim of this work was to study the effects of iodoacetate on the metabolism of the honeybee drone retina. In the superfused retina, iodoacetate only at high concentration (3 mmol·1-1) causes a 77% decrease in the O2 consumption induced by a flash of light. Chromatographic analysis showed that 3 mmol·1-1 iodoacetate strongly inhibited glycolysis in the retinal glial cells and consequently suppressed the biosynthesis of alanine, which is the fuel transferred from the glia to the photoreceptors. However, the synthesis of 14C-alanine from [1-14C]-pyruvate was not affected by iodoacetate. It was therefore surprising to find that superfusion of the retina with 10 mmol·1-1 pyruvate had no protective effect on the decrease in O2 consumption, and that the 14CO2 production from [1-14C]-pyruvate was inhibited 60% by iodoacetate. Also, no protection from the effect of iodoacetate was obtained by adding 10 and 20 mmol·1-1 alanine in the superfusate, even though the transport of 14C-alanine in the photoreceptor cells was not significantly affected by 3 mmol·1-1 iodoacetate. However, exposure to iodoacetate strongly inhibited the production of 14C-glutamate from 14C-alanine. In contrast, the transformation of 14C-proline to 14C-glutamate was not affected by iodoacetate. Indeed, in the presence of iodoacetate, photostimulation caused a decrease in the total concentration of proline and glutamate. It appears therefore that 3 mmol·1-1 iodoacetate inhibits not only glycolysis and, consecutively, the formation of alanine, but also its use in the photoreceptors. Possibly a large intracellular store of proline, whose mitochondrial use was not affected, contributed in slowing down the inhibition of O2-consumption by iodoacetate.