Glucokinase in chicken and mule duck livers

The presence of glucokinase (GK), a critical enzyme controlling glucose homeostasis and particularly liver glucose utilisation in mammals, has long been a matter of debate in avian species as a number of investigators failed to detect glucokinase activity in livers from the chicken and several other avian species. In this study we cloned a partial GK cDNA from mule duck livers and we measured GK-like activity in mule duck and broiler chicken livers under 2 nutritional states. Liver samples from five weeks-old meal-fed male broiler chickens (Ross) were obtained from overnight fasted chickens (FC) and 5 h after an oral saccharose load (6 ml/kg BW of a 50 % saccharose solution) given just before the meal (MC). Liver samples from 15 weeks-old mule ducks were collected after an overnight fast (FD) and 12 hours after the last overfeeding meal (FOD). A partial cDNA (∼600 pb) from duck liver was obtained. It presents 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 % BW respectively for FC and MC (n=8, p<0.05). GK and HK activities levels were similar in FC, respectively 0.88 and 1.00 mU/mg protein. In response to the meal, GK activity increased significantly (+57%) whereas HK decreased (-46 %) in MC. Duck liver weights represented 1.4 % and 7.6 % body weight (BW) respectively for FD and FOD (n=8, p<0.05). In FD livers GK activity was significantly higher than HK activity, respectively 1.76 and 0.63 mU/mg protein. Both activities were significantly increased in fasted overfed ducks (x 2, n=8, p<0.05). In conclusion glucokinase is present in ducks as in chickens and it is nutritionally regulated in avian species as in mammals. Further work will determine whether the higher liver GK activity and GK/HK ratio in FD compared to FC is age related or linked to a high lipogenic capacity of duck liver.