Intracellular water in Artemia cysts (brine shrimp)

摘要

The dormant cysts of Artemia undergo cycles of hydration-dehydration without losing viability. Therefore, Artemia cysts serve as an excellent intact cellular system for studying the dynamics of water-protein interactions as a function of hydration. Deuterium spin-lattice (T1) and spin-spin (T2) relaxation times of water in cysts hydrated with D2O have been measured for hydrations between 1.5 and 0.1 g of D2O per gram of dry solids. When the relaxation rates (I/T1, I/T2) of ²H and ¹⁷O are plotted as a function of the reciprocal of hydration (1/H), an abrupt change in slope is observed near 0.6 g of D2O (or H2 ¹⁷O)/gram of dry solids, the hydration at which conventional metabolism is activated in this system. The results have been discussed in terms of the two-site and multisite exchange models for the water-protein interaction as well as protein dynamics models. The ²H and ¹⁷O relaxation rates as a function of hydration show striking similarities to those observed for anisotropic motion of water molecules in protein crystals.It is suggested here that although the simple two-site exchange model or n-site exchange model could be used to explain our data at high hydration levels, such models are not adequate at low hydration levels (<0.6 g H2O/g) where several complex interactions between water and proteins play a predominant role in the relaxation of water nuclei. We further suggest that the abrupt change in the slope of I/T1 as a function of hydration in the vicinity of 0.6 g H2O/g is due to a change in water-protein interactions resulting from a variation in the dynamics of protein motion.