Role of Histidine in Cataract Formation in Atlantic Salmon (Salmo salar L)

Abstract

Abstract: : Purpose: Elevated levels of dietary histidine have been shown to reduce cataract formation in farmed Atlantic salmon. The aim of this study was to explore the possible roles of histidine and its derivatives in the lens. Since histidine is a buffer at physiological pH, its contribution to lens buffering capacity was examined. In addition, the role of histidine and its analogues in lens osmoregulation was investigated. Methods: A feeding trial was conducted over 29 weeks (5 weeks in freshwater and 24 weeks in seawater). Two dietary groups were established receiving control (11 g kg^-1) and elevated (18 g kg^-1) levels of histidine. Cataract development was monitored by slit lamp throughout the study assessing lens opacity on a scale of 0 – 8. Histidine and histidine derivatives were analysed in whole lens and aqueous humour by HPLC. Lens buffering capacity was measured by titration of lens homogenates with NaOH. Intact lenses from comparable dietary groups were cultured in EMEM at 4 ° C (300 mmol kg^-1). Efflux of histidine and derivatives were measured in isolated lenses after hypo-osmotic (200mmol kg^-1) exposure both by HPLC and by isotopic methods (¹⁴C-histidine). Results: The fish receiving the high histidine diet developed less severe cataracts (average score 1) than the control group (average score 3). Higher levels of histidine (3 fold) and N-Acetylhistidine (10 fold) were found in the lenses of the high dietary histidine group. Lens homogenates from the high histidine group had a higher pH and had a greater buffering capacity. HPLC analysis of NAH content showed a large increase on transfer of the fish to seawater, suggesting a possible role in osmoregulation. Efflux of ¹⁴C- histidine from the salmon lens, measured in vitro, was stimulated after a hypo-osmotic challenge. A 10-fold higher efflux of NAH (measured by HPLC) compared to histidine was observed in lenses similarly exposed. The highest NAH efflux response was seen in the elevated histidine group. Conclusions: The study confirmed the cataract alleviating effect of elevated dietary histidine. Histidine and N-Acetylhistidine in the lens reflected the dietary histidine levels. Differences in osmoregulation and buffering capacity of the lens were related to dietary histidine. Our findings suggest a novel osmoregulatory role for N-Acetylhistidine in the lens of possible importance in cataractogenesis.

Keywords: cataract • nutritional factors