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Fig. 12. Working model of transmural mucosal-to-serosal transport of ³H-L-histidine (His) and ⁶⁵Zn²⁺ across the perfused intestine of the American lobster Homarus americanus. The figure shows three mucosal membrane carrier proteins involved in the movement of these two solutes across the intestine. (1) A relatively specific L-histidine carrier that is not inhibited by L-leucine; (2) a relatively non-specific dipeptide transporter that accepts two histidine molecules (His) linked to a zinc ion in an apparent bis-complex; and (3) a relatively specific zinc transporter that is inhibited by luminal cupric ions (Cu²⁺). Luminal L-leucine (Leu) and glycyl-sarcosine (Gly-Sar) inhibit ³H-L-histidine transport by interacting with the dipeptide carrier in a mixed type inhibition. Luminal copper (Cu⁺ and Cu²⁺) inhibits ⁶⁵Zn²⁺ transport by interacting with the dipeptide carrier in a mixed type inhibition. It is proposed that all interactions observed in this study occur on the brush border membrane (BBM) of intestinal epithelial cells and the mechanisms for efflux of both L-histidine and zinc from the cells to the blood across the basolateral membrane (BLM) are currently unclear.

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