Abstract: The distribution, movement and metabolism of vitamin B₆ (VB₆) compounds in the silkworm, Bombyx mori, were studied in larvae reared on a semisynthetic diet, in which there was no mulberry powder and vitamin-free casein was the protein source. With the increase in the pyridoxine hydrochloride (PN-HCl) content of the diet, pyridoxine (PN) accumulated, and the content of pyridoxal (PL) increased in the larval body. However, the contents of (PLP) and yridoxamine 5'-phosphate (PMP), both of which are the coenzyme forms of VB₆, remained almost constant. PN absorbed by the midgut was first diffused into the hemolymph and then distributed to other organs by active transport. PN was first phosphorylated to pyridoxine 5'phosphate (PNP) by PL kinase, which was then oxidized to PLP by PNP/PMP oxidase in every larval organ except hemolymph. The transportation system of PLP, in which PLP binds to albumin, in the mammalian blood does not exist in silkworms. It is thus thought that the distribution, movement and metabolism of VB₆ compounds are different from those of mammals, and these are the reasons why almost no larvae could reach the next instar when newly molted larvae were reared on a VB₆ deficient diet.

Key words: Bombyx mori, semisynthetic diet, vitamin B ₆, distribution, metabolism