144 Biochimica et Biophysica Acta, 776 (1984) 144-150 Elsevier BBA 72252 HOMEOVISCOUS THEORY UNDER PRESSURE II. THE MOLECULAR ORDER OF MEMBRANES FROM DEEP-SEA FISH * ANDREW R. COSSINS and ALISTER G. MacDONALD ** Department of Zoology, University of Liverpool, P.O. Box 147, Liverpool L69 3BX (U.K.) (Received April 3rd, 1984) Key words: Homeoviscous adaptation," Membrane order," Hydrostatic pressure," Fluorescence polarization; (Deep sea fish) The molecular order of brain and liver membranes isolated from deep sea and continental shelf fish species have been estimated and compared using the fluorescence polarization technique in order to determine whether life in a high pressure habitat is associated with an adjustment of membrane order. Fish were trawled at depths between 200 m and 4000 m, liver and brain membranes were fractionated, and fluorescence polarization was measured at 4°C and ambient pressure. Polarization of the brain myelin fraction provided a statistically significant regression with depth of capture (P < 0.001) with a slope of -0.004 km-i. This change in polarization with depth was sufficient to offset approximately half of the pressure-induced increase in polarization and thus represents the first structural evidence of homeoviscous adaptation to pressure. Polarization of the brain synaptic and liver mitochondriai fraction was not significantly related to depth. This may be due, at least in part, to a high individual variability of polarization compared to laboratory-acclimated freshwater fish. Introduction Hydrostatic pressure has significant effects upon the molecular order of model and natural mem- brane bilayers. A hydrostatic pressure of 1000 atmospheres orders these membranes by an amount equivalent to a 15-25 °C decrease in temperature [1-4]. Nevertheless, animals and bacteria occupy the deep sea at pressures of up to nearly 1100 atmospheres and temperatures less than 4 °C [5]. A reasonable prediction concerning the physio- logical adaptations of abyssal organisms is that the molecular order of their membranes at depth is * The first article in this series is Biochim. Biophys. Acta 775, 141-149. ** Department of Physiology, Marischal College, University of Aberdeen, Aberdeen AB9 1AS, Scotland, U.K. similar to that of the corresponding membranes of comparable shallow water fish. This hypothesis implies that the biochemical composition of the membranes of deep sea fish are modified to offset or compensate for the combined ordering effects of low temperature and high pressure. This is an extension of the concept of homeoviscous adapta- tion [6], which in fish has been studied by compar- ing the membranes of fish from different tempera- tures. In particular brain membranes of fish from the arctic or from hot desert springs [7] and from eurythermal fish acclimated to high or low temper- atures have been compared [8,9]. In both cases, the membranes of the cold-adapted fish were some- what more fluid than those of warm-adapted fish and this difference partially offsets the ordering effects of the cold. The difference in molecular order has been correlated with differences in mem- brane lipid composition [7,8]. 0005-2736/84/$03.00 © 1984 Elsevier Science Publishers B.V.