The pentose phosphate pathway (PPP) produces NADPH, which is utilized for fatty acid synthesis, cholesterol synthesis, steroid biosynthesis, and redox maintenance, and ribose 5-phosphate, which is necessary for nucleic acid synthesis. An alternative pathway of PPP from glucose to 6-phosphogluconate exists in bacteria; however, it remains unclear in animals. In our previous study, SMP30/gluconolactonase (GNL) hydrolyzed glucono 1,5-lactone to gluconate, and gluconokinase (GNK) catalyzed the phosphorylation of gluconate to 6-phosphogluconate in humans and mice. Both SMP30/GNL and GNK proteins were highly expressed in the liver, duodenum, and kidneys. Glucose 6-phosphate dehydrogenase (G6PD) in PPP was highly expressed in epididymal fat, subcutaneous fat, adrenal glands, and brown fat. Overall, the tissue distributions at the protein level were similar between SMP30/GNL and GNK, which was distinct from that of G6PD. To reveal the localization of GNK in mouse tissues, we performed immunohistochemical staining for SMP30/GNL and GNK in alternative PPP and G6PD in PPP. Three C57BL/6J male mice were dissected at 8 weeks of age and paraffin-embedded sections of 17 tissue samples were used for immunohistochemical staining.

Although SMP30/GNL was only stained in the liver, G6PD was stained in all tissues and GNK was stained in almost all tissues (except for cerebrum, epididymal fat, and inguinal subcutaneous fat). GNK and SMP30/GNL were stained in hepatocytes in the liver, whereas G6PD was stained in Kupffer cells. These results suggest that SMP30/GNL and GNK in alternative PPP could function cooperatively in hepatocytes, and the physiological role of alternative PPP might be different from that of PPP in Kupffer cells.