Development and Control of Intestinal and Hepatic Fructokinase

Abstract

Extract: The patterns of development of intestinal and hepatic fructokinase have been studied in the rat, in the rabbit, and in man, and information regarding the mechanism of control of this enzyme in mature organs has been obtained. Fructokinase activity was detectable 4 days before term in fetal rat jejunum (1.0 nmol/min/mg protein) and rose progressively to a plateau 20-fold greater by the 20th day of life (20 nmol/min/mg protein). The enzyme in rat ileum and liver and in rabbit liver behaved similarly (rising from 0.5 to 4.0 nmol/min/mg protein (ileum) and from 0.5 to 8.5 nmol/min/mg protein (liver)). In human fetal liver, there was a threefold rise in fructokinase activity from approximately the 10th to 24th week of gestation (increasing from 2.1 to 7.8 nmol/min/mg protein); jejunal activity rose slightly, but not significantly, during this period. As expected, feeding of mature rats with sucrose after prolonged carbohydrate starvation led to a twofold increase in jejunal (from 6.05 to 13.0 nmol/min/mg protein) and hepatic (from 14.1 to 24.2 nmol/min/mg protein) fructokinase activity. The administration of actinomycin before the introduction of sucrose inhibited the carbohydrate-stimulated rise in fructokinase activity in both organs. Similarly, injection of actinomycin during sucrose feeding produced a significant fall in enzyme activity which greatly exceeded that produced by fasting alone. The data suggest that the substrate-dependent activity of this enzyme may be regulated at the level of transcription.

Speculation: These studies support the hypothesis that the genetic control of certain enzymes involved in carbohydrate metabolism can be influenced by exogenous factors, among them dietary carbohydrate.

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