Muscles fructose metabolism

Fructose-1,6-bisphosphatase deficiency, first describ ed by Baker and Winegrad in 1970, has now been reported in approximately 30 cases. It is more common in women and is inherited as an autosomal recessive disorder. Initial manifestations are not strikingly dissimilar from those of glucose-6-phosphatase deficiency. Neonatal hypoglycemia is a common presenting feature, associated with profound metabolic acidosis, irritability or coma, apneic spells, dyspnea, tachycardia, hypotonia and moderate hepatomegaly. Lactate, alanine, uric acid and ketone bodies are elevated in the blood and urine [11]. The enzyme is deficient in liver, kidney, jejunum and leukocytes. Muscle fructose-1,6-bisphosphatase activity is normal. 

The cell distribution of aldolase B corresponds with its proposed role in gluconeogenesis and D-fructose metabolism.339 Muscle aldolase B acts primarily on D-fructose 1,6-bisphosphate, whereas the aldolase from liver facilitates the cleavage of D-fructose 1,6-bisphosphate and D-fructose 1-phosphate at about equal rates. As a consequence,332,340 the metabolism of D-fructose by way of D-fructose 1-phosphate can readily occur in liver, where it is the primary, metabolic route. 

Fructose metabolism in muscle is illustrated in Fig. 22.2. Fructose is phosphorylated by hcxokina.sc to fructose 6-phosphatc. The fructose 6-phosphate is then used for glycogenesis or, when the glycogen reserves are fuU, energy metabolism via glycolysis. 

Fructose can be metabolized by two routes. In adipose tissue and muscle, hexokinase can phosphorylate fructose to fructose 6-phosphate that then enters glycolysis. In liver, most of the enzyme present is glucokinase not hexokinase and this does not phosphorylate fructose. In this tissue, fructose is metabolized instead by the fructose 1-phosphate pathway. 

Fructose is an abundant sugar in the human diet sucrose (table sugar) is a disaccharide which when hydrolyzed yields fructose and glucose (see Topic Jl) and fructose is also a major sugar in fruits and honey. There are two pathways for the metabolism of fructose one occurs in muscle and adipose tissue, the other in liver. 

The undoubtedly genuine efficacy of branched-chain amino acids cannot be explained merely by the improved metabolism of ammonia, but also by their complex influences on the metabolism of the muscles, liver and brain. The wide range of results may be the outcome of applying various test parameters as well as using different, possibly even unfavourable additives (xylitol, sorbitol, fructose, fats, etc.) Some 18 studies have been evaluated, but an ultimate assessment of the elEcacy of BCAA on HE has not yet been achieved. (124)... 

The initial metabolism of fructt>se tends to occur in liver, not in muscle. The opposite situation holds for glucose. The study reported in Table 4.8, involving human subjects, compares the metabolism of glucose with that of fructose. Fasting... 

Fructose 1-phosphate is further metabolized to dihydroxyacetone phosphate (DHAP) and glyceraldehyde by the hepatic isoform of the enzyme aldolase, which catalyzes a reversible aldol condensation reaction. Aldolase is present in three different isoforms. Aldolase A is present in greatest concentrations in the skeletal muscle, whereas the B isoform predominates in the liver, kidney, and intestine. Aldolase C is the brain isoform. Aldolase B has similar activity for either fructose 1,6-bisphosphate (F16BP) or FIP however, the A or C isoforms are only slightly active when FIP is the substrate. 

Use of D-fructose by muscle cells is still a matter of debate. The results of in vitro experiments indicate rapid, muscular uptake of D-fructose, with both incorporation into glycogen and oxidation to carbon dioxide. However, when D-glucose is added in increasing amounts to muscle-tissue slices, D-fructose utilization decreases. It follows that metabolism of D-fructose in muscle under normal circumstances is limited, owing to competitive inhibition by D-glucose.73... 

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