The identification of extrahepatic "glucokinase" as N-acetylglucosamine kinase

Several research groups have reported the presence of a high Km glucokinase (ATP:D-glucose 6-phosphotransferase, EC 2.7.1.2) in tissues other than adult liver. As shown in this report, protein fractions catalyzing glucose phosphorylation only at high substrate concentrations (100 mM) are indeed found in bovine spleen, rat kidney, human placenta, and newborn rat liver. However, the study of substrate specificities and Michaelis constant values showed that those fractions could be better described as N-acetylglucosamine kinase (ATP:acetamide-2-deoxy-D-glucose-6-phosphotransferase, EC 2.7.1.9) which, in addition to N-acetylglucosamine (Km = 0.066 mM), can also phosphorylate glucose although with very high Km values (370 mM). Furthermore, a homogeneous preparation from bovine spleen was able to phosphorylate both N-acetylglucosamine and glucose. An immune serum against bovine spleen N-acetylglucosamine kinase did not cross-react with purified hexokinases or with glucokinase from rat. However, it was able to remove the putative "glucokinases" from extracts of rat kidney, newborn rat liver, and one of two electrophoretic bands of liver "glucokinase." It is proposed that any report of extrahepatic glucokinase should explicity rule out N-acetylglucosamine kinase as the enzyme being described.