Expression of the glucokinase gene in mule duck liver and glucokinase activities in chicken and mule duck livers

The presence of glucokinase (GK), a critical enzyme controlling glucose homeostasis, particularly liver glucose utilization in mammals, has long been a matter of debate in avian species because a number of investigators have failed to detect GK activity in the livers of chickens and several other avian species. In this study, we cloned a partial GK cDNA from mule duck livers and measured GK-like activity in the livers of mule ducks and broiler chickens under 2 nutritional states. Liver samples from 5-wk-old meal-fed male broiler chickens (Ross) were obtained from overnight-fasted chickens (BC) and 5 h after an oral saccharose load (6 mL/kg of BW of a 50% saccharose solution) given just before the meal (BS). Liver samples from 15-wk-old mule ducks were collected after an overnight fast (DC) and 12 h after the last overfeeding meal (DO). A partial cDNA (similar to 600 bp) was obtained from duck livers. It presented 99% identity with chicken partial GK cDNA (gi 44888789) and 82% identity with human GK (gi 15967158). Chicken liver weights represented 1.8 and 3.3% of BW, respectively, for BC and BS (n = 8, P < 0.05). Glucokinase and low-Michaelis constant hexokinase (HK) activity levels were similar in BC (respectively, 0.88 and 1.00 mU/mg of protein). In response to the meal load, GK activity increased significantly (+57%), whereas HK decreased (-46%) in BS. Duck liver weights represented 1.4 and 7.6% of BW, respectively, for DC and DO (n = 8, P < 0.05). In DC livers, GK activity was significantly higher than HK activity (respectively, 1.76 and 0.63 mU/ mg of protein). Both activities were significantly increased in DO (2 times, n = 8, P < 0.05). In conclusion, GK is present in ducks as well as chickens, and it is nutritionally regulated in avian species as well as in mammals. Further work will determine whether the higher liver GK activity and GK:HK ratio in DC compared with BC is related to age or BW or linked to the high lipogenic capacity of the duck liver.