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International audience; Glycogen storage disorder type III (GSDIII) is a rare inborn error of metabolism due to loss of glycogen debranching enzyme activity, causing inability to fully mobilize glycogen stores and its consequent accumulation in various tissues, notably liver, cardiac and skeletal muscle. In the pediatric population, it classically presents as hepatomegaly with or without ketotic hypoglycemia and failure to thrive. In the adult population, it should also be considered in the differential diagnosis of left ventricular hypertrophy or hypertrophic cardiomyopathy, myopathy, exercise intolerance, as well as liver cirrhosis or fibrosis with subsequent liver failure. In this review article, we first present an overview of the biochemical and clinical aspects of GSDIII. We then focus on the recent findings regarding cardiac and neuromuscular impairment associated with the disease. We review new insights into the pathophysiology and clinical picture of this disorder, including symptomatology, imaging and electrophysiology. Finally, we discuss current and upcoming treatment strategies such as gene therapy aimed at the replacement of the malfunctioning enzyme to provide a stable and long-term therapeutic option for this debilitating disease.

The current pork slaughter process is primarily optimized to reduce cooler shrink and the incidence of PSE pork. Elimination of the halothane gene and improved preslaughter handling have decreased the incidence of PSE pork and improved the water-holding capacity of the muscle; however, the chilling process has not been optimized to accommodate these changes. The hypothesis that stepwise chilling could improve tenderness without compromising water-holding capacity was tested in this study. The stepwise chilling treatments were composed of a rapid chilling to 10 or 15 degrees C (in a chilling tunnel) and a 6-h holding period at 10 or 15 degrees C, followed by rapid chilling to 4 degrees C. Both treatments were compared directly with a chilling treatment that simulated conventional tunnel chilling; one carcass half from each pig was allocated to a stepwise chilling treatment, whereas the other carcass half was allocated to the control treatment. A total of 42 pigs were slaughtered on 6 slaughter days. Biopsies were collected for analysis of glycogen degradation and glycogen debranching enzyme activity from slaughter until 72 h postmortem, and samples for color, sarcomere length, drip loss, Warner-Bratzler shear force, and sensory analysis were removed from the carcass 24 h postmortem. Substantial temperature differences were obtained during the holding period between the stepwise and conventionally chilled carcass halves. These had almost, but not completely, disappeared by 22 h postmortem, and although the differences were small, pH was significantly (P 0.05) because of the smaller temperature differences in these muscles. The improvements in tenderness could be solely attributed to the increased proteolysis postmortem in the stepwise-chilled carcasses, with the greater temperatures favoring proteolytic enzymes involved in muscle protein degradation. Furthermore, the results for glycogen metabolism successfully revealed that both pro- and macroglycogen contributed to the energy generation in postmortem muscles, with degradation of both forms early postmortem.

AIMS: Gel electrophoresis revealed a band of molecular weight approximately 160 000 Da associated with the skeletal muscle sarcoplasmic reticulum (SR) vesicle preparations. This investigation sought to examine glycogen debranching enzyme associated with skeletal muscle SR. METHODS: Sarcoplasmic reticulum samples were also taken from muscle whose glycogen content had been reduced either via stimulation of the sciatic nerve or alpha-amylase treatment of muscle homogenates. RESULTS: The stimulation protocol reduced whole muscle glycogen by 86% (7.4 +/- 0.4 vs. 1.0 +/- 0.3 microg mg(-1) wet mass, P < or = 0.05). Glycogen associated with the SR was reduced by 82% in the stimulation protocol (533 +/- 82 vs. 96 +/- 7 microg mg(-1) protein) and by 94% in alpha-amylase treatment (493 +/- 11 vs. 29 +/- 2 microg mg(-1) protein), respectively. Gel electrophoresis and Western blots revealed that the content of glycogen debranching enzyme was reduced by approximately 53% as a result of muscle stimulation and by approximately 46% in alpha-amylase treatment (P < or = 0.05). In addition, glycogen debranching enzyme activity was reduced by 61% in stimulated samples compared with control (20.3 +/- 1.0 vs. 8.0 +/- 1.2 nmol mg(-1) min(-1), respectively), a value consistent with reductions observed from gel electrophoresis and Western blots. CONCLUSION: These results confirm that similar to glycogen phosphorylase, glycogen debranching enzyme is associated with the skeletal muscle SR and is dissociated under exercise conditions.

The degradation of glycogen progresses by the co-operation of two enzymes: glycogen phosphorylase (phosphorylase) and glycogen debranching enzyme (GDE). We studied the effect of temperature (4-42°C) and salt concentration (0-3% NaCl) on bovine M. longissimus dorsi GDE activity. GDE activity (n=4) decreased significantly with decreasing temperature from about 40-4°C. GDE exhibited 52% activity at 25°C and 11% at 4°C compared to its optimum activity measured at 39°C. In rapidly chilled meat, the reduction in GDE activity may substantially delay the rate of glycolysis. However, residual GDE activity at 4°C seems sufficient to enable the attainment of normal ultimate pH if the available time is long enough. An increase in salt concentration from 0% to 2% and to 3% induced a significant (P0.001) increase in the ultimate pH of ground bovine meat (n=6), but showed no effect on GDE activity.

The activity of glycogen debranching enzyme (GDE) was studied in relation to pH value and temperature in porcine masseter and longissimus dorsi muscles. A glycogen limit dextrin was used as the substrate for GDE, and the enzyme was derived from raw meat extracts. In both muscles, the pH only weakly affected on activity of GDE at the pH values found in carcasses post-slaughter. However, the activity of GDE decreased strongly (P < 0.001) when the temperature decreased from values of 39 °C and 42 °C found just after slaughter to values of 4 °C and 15 °C found during cooling. In both muscles the activity of GDE began to fall at temperatures below 39 °C and was almost zero when the temperature decreased to below 15 °C. Thus, the activity of GDE may control the rate of glycogenolysis and glycolysis, but does not block rapid glycolysis and pH decrease when the temperature is high. This may be important in PSE meat, where the pH decreases rapidly at high temperatures, but rapid cooling could limit the activity of GDE and thus glycogenolysis. As expected, GDE was more active in the light longissimus dorsi muscle than in the dark masseter muscle.

Glycogen debranching enzyme contains two catalytic activities (4-alpha-glucanotransferase and amylo-1,6-glucosidase) on its single polypeptide chain, and they are affected differently by the binding of oligosaccharides. Glucose, maltose, and maltotriose are competitive inhibitors of the amylo-1,6-glucosidase activity measured by the hydrolysis of alpha-glucosyl fluoride, whereas saccharides with four or more glucose units are activators of the same activity, showing apparent "uncompetitive" kinetics. This suggests that they do not bind until the alpha-glucosyl fluoride is bound. In either case the potency of the effect increases with the length of the oligosaccharide chain. On the other hand, all oligosaccharides tested (maltose to maltohexaose, alpha-cyclodextrin, and beta-cyclodextrin) are competitive inhibitors of the transferase activity and also cause a decrease in the intrinsic fluorescence, both functions again increased by chain length, thus indicating that these saccharides do bind to the free enzyme. These interesting results can be reconciled if the extended main chain resulting from the transferase reaction has to be reoriented into a different binding mode in order to position the alpha-1,6-linked side-chain glucose into the correct position for the glucosidase reaction. Therefore, activating oligosaccharides behave kinetically as if they had not been previously bound. It is concluded that the main chain of the natural limit dextrin substrate has a different mode of binding for the two catalytic reactions in order to position properly first the maltotetraosyl side chain in the transferase catalytic site and then the glucosyl side chain in the glucosidase catalytic site.(ABSTRACT TRUNCATED AT 250 WORDS)