Thomas H. Claus

1 Enzymes of the Fructose 6-Phosphate-Fructose 1, 6-Bisphosphate Substrate Cycle

Publisher Summary This chapter discusses the regulation of enzyme activities of 6-phosphofructo-1-kinase, fructose-1,6-bisphosphatase, and 6-phosphofructo-2-kinase-fructose-2,6-bisphosphatase, with particular emphasis on the role of phosphorylation. Only in a few instances, phosphorylation–dephosphorylation has been shown to regulate an enzyme involved in the interconversion between fructose 6-phosphate and fructose 1,6-phosphate in a physiologically relevant way. The best example is regulation of hepatic 6-phosphofructo-2-kinase-fructose-2,6-bisphosphatase. The phosphorylation of this enzyme by the cyclic AMP-dependent protein kinase with resulting changes in the enzyme activities has been well characterized in vitro using purified preparations. Similar reciprocal changes in the enzyme activities have been demonstrated both in vivo and in isolated liver systems in response to elevated levels of cyclic adenosine monophosphate (AMP). There is also preliminary evidence that 6-phosphofructo-2-kinase in yeast and heart is regulated by phosphorylation. However, in neither case has the 6-phosphofructo-2-kinase been purified to homogeneity and its regulatory properties and in vitro phosphorylation studied in detail. With regard to regulation by phosphorylation, the enzymes in yeast and heart are different from that found in the liver. Mammalian 6-phosphofructo-1-kinase from the heart and skeletal muscle has been shown to contain covalently bound phosphate in vivo and to be substrates for the cyclic AMP-dependent protein kinase in vitro . It has been reported in the chapter that the muscle enzyme from Ascaris suum is activated by phosphorylation catalyzed by the cyclic AMP-dependent protein kinase. In Saccharomyces cerevisiae, cyclic AMP-dependent phosphorylation of fructose-1,6-bisphosphatase appears to play a role in catabolite repression leading to a decrease in enzyme activity and acting as a signal for proteolytic degradation.

Changes in fructose-2,6-bisphosphate levels after glucose loading of starved rats

Fructose-2,6-bisphosphate levels in freeze-clamped livers of starved rats were 0.5 nmol/g liver. Oral administration of 1 g glucose per kg body weight to starved rats increased glycogen levels from 4 mg/g liver to 13.5 mg/g in 2 hr but did not significantly alter fructose-2,6-bisphosphate levels. The low level of this effector is consistent with an active gluconeogenic process and the results support the hypothesis that carbon atoms for glycogen synthesis can be derived from 3-carbon precursors via this pathway, even in the presence of glucose.

Changes in key regulatory enzymes of hepatic carbohydrate metabolism after glucose loading of starved rats

When glucose was given to starved rats there was an increase in both 6-phosphofructo 2-kinase and pyruvate kinase activity and a decrease in fructose 2,6-bisphosphatase activity 30 min and 60 min later. These changes were accompanied by an increase in glycogen deposition and by modest, but significant increases in fructose 2,6-bisphosphate levels at the same time. Metabolite measurements indicated that flux through 6-phosphofructo 1-kinase and pyruvate kinase were increased. These results suggest that although glycogen deposition may occur via the gluconeogenic pathway, glycolysis is activated at the same time by changes in the phosphorylation state of key regulatory enzymes as well as by the small rise in fructose 2,6-bisphosphate.

Fructose 2,6-bisphosphate levels are elevated in livers of genetically obese mice

Abstract Fructose 2,6-bisphosphate levels in freeze-clamped livers of C57BL 6J ob ob mice were 6-fold higher than the level in their lean (+/?) littermates. Overnight starvation reduced the hepatic level of this unique sugar diphosphate to 0.2 nmol/g in both the obese and lean mice. The elevated level in the obese mouse is consistent with the hyperinsulinemia of these animals.

Chapter 20. β3-Selective Adrenergic Receptor Agonists

Publisher Summary The clinical utility of β 3 -AR agonists depends on both their efficacy and on their selectivity. The presence of significant β 1 - and β 2 -AR related side-effects has been responsible for stopping the development of several compounds and highlights the importance of optimizing β 3 -selectivity against human receptors. This has contributed to the subdivision of β-adrenergic receptors (β-AR) into β 1 - and β 2 -AR has led to the development of β 1 - and β 2 -AR antagonists and agonists, as well as nonselective β 3 -AR antagonists, that have been useful for the treatment of cardiovascular disease and asthma. A third subtype of β-AR has led to the development of β 3 -AR agonists that may prove useful in the treatment of various metabolic diseases. A common feature of atypical or β 3 -AR is that they mediate responses that are insensitive to the most standard β-AR antagonists. They are also more responsive than β 1 - or β 2 -AR to the novel agonists. The selectivity of β 3 -AR agonists has typically been determined by functional assays in adipocytes or colon, atrium, and trachea or uterus. More recently, a series of β 3 -AR agonists has been ranked by their ability to stimulate cyclic adenosine monophosphate (AMP) production in Chinese hamster ovary cells that have been transfected with the human β 1 -, β 2 -, or β 3 -AR. Two aryloxypropranol-aminotetralines have recently been reported to be selective antagonists for the β 3 -AR.