Circadian changes of D-alanine and related compounds in rats and the effect of restricted feeding on their amounts

The circadian changes of D-alanine (D-Ala), an intrinsic D-amino acid found in mammals,were investigated in rats with diurnal and nocturnal habits, and the profiles were compared to those of L-Ala, other D-amino acids and several hormones. Determination Of D-Ala in the rat plasma, pancreas and anterior pituitary gland was carried out using a sensitive and selective two-dimensional HPLC system combining a micro-ODS column and an enantioselective column after fluorescence derivatization with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F). The amount of D-Ala was high during the sleeping period and low during the active period in rats with both diurnal and nocturnal habits, indicating for the first time that the D-Ala is closely related to the activity rhythm of animals. In contrast, L-Ala and other D-amino acids did not show any clear circadian changes. The circadian change of D-Ala inversely correlated with that of the plasma insulin level in rats with both diurnal and nocturnal habits. Considered together with our previous findings that D-Ala is localized in the insulin secreting beta-cells in the rat pancreas, it is strongly suggested that D-Ala has some functional relationships to insulin in mammals. (c) 2008 Elsevier B.V. All rights reserved.