Hexokinase in Conducting Tissues1

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Abstract
The localization and properties of hexokinase have been studied in the conducting bundles of leaf petioles of sugar-beet. It has been shown that 12 to 20 per cent of hexokinase activity is found in the mitochondrial fraction and more than 10 per cent in the microsomal fraction. The remaining hexokinase activity is concentrated in the soluble fraction of the cell. The properties of hexokinase associated with the structural elements and with the soluble portion of the cell are different. The hexokinase in mitochondria and microsomes is non-specific; it phosphorylates both glucose and fructose but possesses a much greater affinity for glucose. Consequently the phosphorylation of fructose in these fractions is almost completely inhibited in the presence of glucose, whereas fructose has only a relatively small suppressive effect on glucose phosphorylation. In the soluble fraction of the cells which corresponds to the cytoplasm, the phosphorylation of each of the hexoses occurs independently of the presence of the other sugar. The phosphorylation of fructose in this fraction is intensified in the presence of K+ ions (activator of fructokinase). On this basis it is suggested that the soluble fraction contains specific fructo-kinase along with non-specific hexokinase. The results are compared with those obtained earlier on the competition of these sugars during their transport into the cells of conducting bundles. Here glucose is a strong inhibitor of the fructose transport, while the latter only slightly affects the influx of glucose. This gives grounds for the belief that non-specific hexokinase, localized on the membranes, can take part in the transport of hexoses into the cell.

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