Amino Acid Uptake and Metabolism by Larvae of the Marine Worm Urechis caupo (Echiura), a New Species in Axenic Culture

¹ Department of Biological Sciences, University of Southern California, Los Angeles, California 90089-0371

Axenic (bacteria-free) larval cultures of the marine echiuran worm, Urechis caupo, were reliably obtained by aseptically removing gametes directly from the gamete storage organs. Trochophore larvae only removed neutral amino acids from seawater as measured by high-performance liquid chromatography (HPLC). There was no detectable uptake, as measured by HPLC, of acidic or basic amino acids. Kinetic analysis showed that the transport system for alanine in 4-day-old larvae had a K_t of 4-6 µM and a J_max of 9-10 pmol larva^-1 h^-1. Following a 50-min exposure, the majority of the radioactivity (95%) from ¹⁴C-alanine was found in the trichloroacetic acid-soluble fraction. Very little label appeared as acid-insoluble material, and there was no detectable lipid biosynthesis from ¹⁴C-alanine. Approximately 12% of the total alanine transported was released in the form of ¹⁴CO₂. Thin-layer chromatography of intracellular free amino acid pools demonstrated that aspartic acid and glutamic acid were radiolabeled from the alanine precursor. A comparison of the energy acquired from the transport of alanine, with the metabolic rate of 4-day-old larvae, revealed that 51% of the metabolic demand could be provided by the transport and complete catabolism of this single amino acid at a concentration of 595 nM in seawater.

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