Water, sodium, potassium, ATP, amino acids, and sugars are not uniformly distributed in Rana pipiens oocytes. Concentration differences exist between nucleus (germinal vesicle) and ooplasm and between animal and vegetal ooplasmic regions. The mechanisms responsible for these differences were investigated using intracellular reference-phase (iRP) analysis. The iRP is an artificial "organelle" that has the solvent properties of a dilute salt solution and is in diffusional equilibrium with water and solutes present in other cellular compartments. Ooplasm/iRP solute distributions show that ooplasm differs from ordinary aqueous solutions--exhibiting both solute exclusion and solute binding. Yolk platelets are an important cause of this behavior, largely because their proteins are present as hydrate crystals, which are rich in anionic sites and which interact intensely with associated water. Because of yolk's abundance, it obscures the solvent and binding properties of ooplasmic ground substance. The oocyte nucleus is yolk and organelle free and the nuclear envelope is readily permeable. Consequently, nucleus/iRP solute concentration differences reflect the binding and solvent properties of nuclear ground substance. Nucleoplasm binds approximately 19 meq of potassium. Furthermore, the monosaccharides, 3-O-methylglucose, L-glucose, and D-xylose, are selectively excluded, their nucleus/iRP concentration ratios averaging about 0.7; ratios for other solutes studied are unity. We interpret monosaccharide exclusion to mean that nuclear ground substance water is different in its "instantaneous" structure from ordinary saline water. Because of this difference, hydrogen bond interaction between nuclear water and certain sterically restricted solutes, of which ringed monosaccharides are examples, is reduced. Some implications of modified ground substance water and selective solute exclusion are discussed.

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