4-Hydroxyisoleucine: effects of synthetic and natural analogues on insulin secretion

4-Hydroxyisoleucine, a peculiar amino acid extracted from fenugreek seeds and never found in mammalian tissues, exhibits interesting insulinotropic activity. To investigate the structural requirements for this stimulating effect, the insulinotropic activity of the major isomer (2 S,3 R,4 S) of 4-hydroxyisoleucine, in the presence of 8.3 mM glucose, was compared to that of (1) its minor isomer (2 R,3 R,4 S) (2) its lactone form, (3) classical structurally related amino acids, and (4) synthetic monomethylated analogues. In the isolated, ex vivo, perfused rat pancreas, only the major isomer of 4-hydroxyisoleucine (200 μM) potentiated insulin release. On incubated isolated rat islets, the threshold concentration for a significant increase ( P