The present invention relates to (1) a polypeptide containing the same or substantially the same amino acid sequence as the amino acid sequence represented by SEQ ID NO:7 (A chain), (2) a polypeptide containing the same or substantially the same amino acid sequence as the amino acid sequence represented by SEQ ID NO:8 (B chain), (3) a polypeptide wherein the A and B chains are bonded to each other via disulfide bonds (2 chains) or its salt, (4) DNAs encoding the same, etc. These polypeptides and DNAs encoding the same can be used for the diagnosis, treatment, prevention, etc. for diseases, including abnormalities (e.g., diabetes mellitus, etc.) in metabolic regulation (sugar metabolism, lipid metabolism, etc.) of energy sources such as carbohydrates.