The Effect of Zinc on the Transmural Transport of L-H-Histidine in the Intestinal Epithelium of the American Lobster, Homarus Americanus

vi, 44 leavesIt has been demonstrated that in rat erythrocytes, L-histidine enhances the transport of zinc (Aiken et al., 1992). These experiments also indicated that histidine uptake in rat erythrocyte is partially sodium dependent and shows inhibition by leucine. These observations suggest the involvement of the L-system carrier which is capable of transporting histidine with the participation of sodium ions. Similar effects have been shown to occur in the rat intestine (Wapnir et al., 1983). Other work has been conducted testing zinc absorption in rats and the effects of amino acids (histidine, cysteine, tryptophan and proline) on this process (Wapnir & Stiel, 1986). This study determined that histidine assisted in the transport of zinc in the jejunem and ileum of the rat. Further evidence shows that metals stimulate amino acid transport in crustacean hepatopancreatic cells. Monteilh-Zoller, Zonno, Storelli, and Ahearn (1999) showed that zinc doubled the transmembrane transfer of L-proline in lobster hepatopancreatic brush border membrane vesicles. The enhanced uptake of the amino acid in the presence of the metal was found to be due to increased 3H-L-proline maximal transport velocity (i.e. Jmax), rather than due to a change in binding affinity (i.e. Kt) induced by the metal. The L-proline transport occurred by way of a specific transport protein, the IMINO system. These studies indicate that zinc and histidine form a complex that is transported together. It is thus postulated that zinc enhances the transport of histidine. In this study, the effect of zinc on the transmural flux of L-histidine across the intestine of the Atlantic lobster (Homarus americanus) was investigated. Previous studies of amino acid transport involved alanine, a non-essential amino acid, which was largely metabolized by the tissue (Wyban, Ahearn & Maginniss, 1980). However, in this study, L-histidine, an essential amino acid for the growth of lobsters and other crustaceans, was the nutrient considered (Factor, 1995). Experiments support the hypothesis that the uptake of L-histidine is increased in the presence of zinc (Ahearn, H.R.H. et al, 2000; Liou & Ellory, 1990). The first portion of this project determined the net transmural flux of 3H-L-histidine at five different concentrations, in the presence and absence of a defined zinc concentration. Unidirectional transepithelial transport was measured across the intestine as a function of time. These measurements established that transport is likely carrier-mediated, as a saturation of a carrier molecule will limit transport. The second portion of the described research studied the influence of varying the concentration of zinc at a constant concentration of 100 µM L-histidine. The concentrations of zinc ranged from 5 µM to 250 µM. The optimal concentrations of zinc, which produced maximal carrier-mediated activation, were determined to be between 25 µM and 50 µM