Vitamin A isomers in some eucaridan Crustacea

Abstract

Earlier work in this laboratory showed that the biological activity of vitamin A extracts from some marine Crustacea was lower than that indicated by the Carr–Price test. It was suggested that this discrepancy might have been due to the presence of cis-isomers of vitamin A, which give the same quantitative colour reaction in the Carr–Price test but are biologically less active than all- trans vitamin A. In this paper, the isomer composition of the vitamin A of two euphausiid Crustacea, Meganyctiphanes norvegica (M. Sars) and Thysanoessa raschii (M. Sars), and of three decapod Crustacea, Pandalus bonnieri Caullery , P. borealis Kroyer and P. montagui Leach, is reported. The percentages of ‘fast’ (all- trans and 9- cis vitamin A) and ‘slow’ (13- cis , 9:13-di- cis , neo- b and neo- c vitamin A) reacting isomers were calculated from the rate of combination of vitamin A preparations from these species with maleic anhydride in benzene solution. Hindered cis -isomers were detected by their rapid isomerization with iodine, which results in an increase of the wavelength of the ultra-violet absorption maximum and a large increase in extinction, and by a high value for the ratio (maximum extinction in the Carr-Price test)/ (maximum extinction in the ultra-violet). The retinenes of these vitamin A isomers were allowed to react with opsin in the dark and the neo- b retinene present was measured by the formation of rhodopsin. In all five species studied, the hindered cis -isomer, neo- b vitamin A, was the most abundant form and constituted 53 to 90 % of the total vitamin A. Another hindered cis -isomer, neo- c vitamin A, not so far reported from natural sources, was isolated from extracts of M . norvegica and T . raschii by fractional chromatography. It constituted about 5% of their total vitamin A. The remainder of the vitamin A was mostly in the all- trans form. The calculated biological activities of these mixtures of isomers agreed well with the results of biological assays with rats. As the vitamin A is located in the eyes of these Crustacea, it is possible that it has a visual function. The significance of these findings in relation to the vitamin A food chain in the sea is discussed, and it seems probable that the predators of the Crustacea are able to convert neo- b vitamin A into the all - trans form.