Different physiological functions of free D- and L-alanine in three body parts of the intertidal sipunculid Phascolosoma arcuatum

Free D- and L-alanine contents were comparable in the body wall and introvert cum retractor muscles of Phascolosoma arcuatum. In contrast, the content of free D-alanine in the internal organs was twice that of free L-alanine. Since alanine aminotrans-ferase from P. arcuatum was L-alanine specific, D-alanine appeared to be synthesized from L-alanine through the action of alanine racemase. Alanine racemase activity was higher in the D-alanine-forming direction in the three body parts of P. arcuatum. In addition, the ratio of D→L/L→D racemase activity in the internal organs was the lowest among the body parts studied. These results indicate that free D-alanine might be of lesser importance than the free D-isomer to the internal organs as compared to the body wall and introvert cum retractor muscles. Indeed, L-alanine inhibited pyruvate kinase from the body wall and introvert cum retractor muscles but had no effect on the pyruvate kinase from the internal organs. Furthermore, the activity of alanopine dehydrogenase present in the internal organs was significantly lower than those of the body wall and introvert cum retractor muscles. L-Alanine was an essential substrate for alanopine formation in the body wall and introvert cum retractor muscles during hypoxia since alanopine dehydrogenases from these body parts were L-alanine specific. When P. arcuatum was confronted with hypo-osmotic stress, the free D-alanine/total free alanine ratio in the internal organs increased approximately from 0.6 to 0.8 as the total free alanine content decreased. In comparison, those ratios in the body wall and introvert cum retractor muscles remained relatively constant. It was concluded that D- and D-alanine had different physiological functions in the three body parts of P. arcuatum.