L-Proline Transport Systems of Starfish Pyloric Caeca

Purified brush-border membrane vesicles (BBMV) of starfish [Pycnopodia helianthoides (Brandt)] pyloric caecal epithelium were prepared by magnesium precipitation in order to characterize the possible role of this organ in amino acid transport. L-[³H]proline uptake by these vesicles was Na⁺-dependent and greater at pH7.5 than at pH5.5. L-Pipecolate was a competitive inhibitor of L-proline influx into these BBMV, exhibiting a K_i value of 0.02 mmol l⁻¹. The amino acid inhibitors, L-pipecolate, L-alanine and L-leucine were used as test substrates to block L-proline influx by the IMINO, NBB and L transport systems, respectively, in order to estimate the contribution of each process to total L-proline entry into pyloric caecal cells. The carrier-mediated transport constants for L-proline transfer by these three systems were: K_t=0.18mmoll⁻¹ (IMINO), 0.13mmoll⁻¹ (NBB) and 0.21 mmol T¹ (L); J_max= 1310 pmol mg⁻¹ protein 30 s ⁻¹ (IMINO), 360 pmol mg⁻¹ protein 30 s⁻¹ (NBB) and 470pmolmg⁻¹protein30s⁻¹ (L). L-Proline influxes through both the IMINO and NBB systems were sigmoidal functions of the external [Na⁺], while transfer by the L system was Na⁺-independent. Multiple sodium ions (e.g. 2 or 3 Na⁺/L-proline) appear to be associated with L-proline transport by both Na⁺-dependent transport systems, but the nature of this association (i.e. activation or energization) is unclear. Results suggest that starfish pyloric caecal epithelium possesses a similar array of L-proline transport proteins to those found in similar cell types of mammalian intestine or kidney, providing tentative support for an absorptive function for this organ.