Investigations of the Enzymes Involved in the Fructose Breakdown in the Cattle Lens

Abstract

By introducing fructose into the glycolysis, it is possible to stimulate ATP formation. As is the case in animal experiments, in human lenses, too, the first step is the phosphorylation to fructose-1-phosphate via the enzyme ketohexokinase. The present investigation deals with the question whether enzymes present in the lens are responsible for the further steps in fructose degradation. Particularly the aldolase isoenzyme C splits fructose-1-phosphate into glyceraldehyde and dihydroxyacetone phosphate in the same way as in glucose catabolism. Dihydroxyacetone phosphate can further be directly degraded and thus utilized to ATP formation. From glyceraldehyde, glycerol (aldose reductase) or glycerate (aldehyde dehydrogenase) can be formed. The presence of triosekinase, which phosphorylates glyceraldehyde directly to glyceraldehyde-3-phosphate, could only be determined in the lens tissue of young animals. The presence of glycerokinase (glycerol → glycerophosphate) could not be verified. Thus, in the old lens tissue 1 ATP molecule net per fructose molecule can be formed. In older age, the glucose breakdown is limited by hexokinase and phosphofructokinase, so that the glucose, after transformation via the sorbitol pathway to fructose, can also be utilized for the energy metabolism.

Fructose, Fructose 1-Phosphate, and Glyceraldehyde Breakdown in Carbohydrate Metabolism

“…Besides aldose reductase, mainly aldehyde dehydrogenase competes for the glyceraldehyde breakdown, while triosekinase activity is negligible [Ohrloff et al, 1981Jedziniak and Rokita, 1981]. This finding is also supported by a lack in lactate increase after glyceraldehyde incuba tion.…”

Glyceraldehyde Is Present in Rat Lens and Its Level Is Increased in Diabetes Mellitus

“…In the lens, as distinct from the liver, fructose is synthesized from glucose by the polyol pathway, which consists of 2 reactions by aldose reductase and sorbitol dehydrogenase [12] . It is then changed to fructose 1-phosphate by ketohexokinase, followed by its split into glyceraldehyde and dihydroxyacetone phosphate by aldolase type C [5] . Our finding that the aldose reductase inhibitor M16287 reduced the glyceraldehyde content in lenses cultured in a high concentration of glucose for 6 days ( fig.…”

“…An average molar concentration of glyceraldehyde of 1.7 M can be calculated from this value, provided that the average water content in rat lenses is 60% (v/w) and both glyceraldehyde and water are present homogeneously throughout the lens. It has been reported, however, that the distribution of various components, for example enzymes [5] , glutathione [14] and NAD [15] , is rather heterogeneous in the lens, which consists of epithelial cells, fiber cells and nucleus. It is highly possible, therefore, that glyceraldehyde concentrations in some parts of the lens can reach 5-10 M or even more.…”

Comparison of Aldose Reductase Inhibitors by Determination of IC50with Bovine and Rat Lens Extracts

“…While studying the catabolism of fructose and its metabolites [Ohrloff et al, 1981[Ohrloff et al, , 1983, the question arose whether the sor bitol pathway with its transformation of glu cose to fructose and the further degradation initiated by the enzyme ketohexokinase Hockwin, 1973. 1982;Ohrloff et al, 1974 might be of any impor tance in respect to the total energy metabo lism of the lens.…”