Influence of some environmental variables on the ascorbic acid status of mullet, Mugil cephalus L., tissues

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Abstract
The seasonal fluctuations in the ascorbic acid (AsA) and ascorbic acid 2‐sulphate (AsA 2‐sulphate) content of mullet, Mugil cephalus, tissues were examined. Ascorbic acid concentrations in brain, gill and hepatic tissues showed seasonal changes, but the pattern of AsA fluctuations in each tissue differed. The AsA content of mullet brains decreased during the summer, whereas hepatic AsA concentrations increased during this period and were maximal by the end of June. Hepatic AsA reserves declined after environmental water temperatures dropped below 18°C and reached a minimum (20 μg g−1) by the end of January. Greatest fluctuations in AsA content occurred in gill tissues, which had a four‐fold range of tissue concentrations. There were also seasonal changes in the AsA 2‐sulphate content of brain and hepatic tissues. These differences among mullet tissues in the seasonal patterns of AsA content may be due to diverse effects of environmental variables on tissue AsA reserves. The ability of hepatic and renal tissues of mullet and several other teleost species to synthesize AsA was also investigated. L‐gulonolactone oxidase activity was detected in all the species examined, but in all cases the biosynthetic capacity was less than a seventh that in goldfish, Carassius auratus, livers. Mullet appear to have only a limited capacity to synthesize AsA.