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SUMMARY: An osmotic barrier for phosphate very near the visible surface of the chick-embryo cell appears to regulate the reciprocal exchange of inorganic ortho- phosphate between the medium and a component of the acid-soluble inorganic phosphate of cells kept in monolayer culture. Some non-reciprocal transfer of inorganic phosphate occurs, which may or may not be due to cell damage, and the apparent phosphate-impermeable volume decreases by half after 3 hr. contact with phosphate at 0'. At 2' the exchange is at least 97 yo inhibited. The phosphate- impermeable volume after cell rupture is less than 10% of the intact-cell value, when internal phosphate is released in a form shown by mild separation to be mostly inorganic orthophosphate. Cells lose internal phosphate more slowly in absence of external phosphate, and the addition of external phosphate increases the loss rate to a maximum at physiological concentrations and higher. Such a barrier provides information on the nature of the cell surface, and shows that adsorption, penetration and release of vesicular stomatitis virus occurs without any detectable damage to the surface controlling the phosphate-exchange rate. Workers generally agree (e.g. Kamen & Spiegelman, 1948; Sacks, 1948; Mitchell & Moyle, 1953) that concentration of inorganic phosphate into cells does not depend much on simple diffusion alone, but also involves enzymic mechanisms which are possibly phosphorylative. The major uncertainties reside in whether (a) these are exclusively surface mechanisms, so that internal orthophosphate is free within an acid-labile osmotic barrier, or whether (b) the phosphate diffuses freely, to be immobilized enzymically by some in- tracellular acid-labile matrix. Sacks (1948) believed the evidence indicated possibility (a) for animal cells, but he considered only the alternative of simple physical diffusion (i.e. with no enzymic mechanism) rather than (b). Similar possibilities have been discussed by Mitchell (1953) for Mimococcus pyogenes (Staphylococcus aureus) under the titles of (a) exchange-diffusion and, (b) exchange adsorption, although simple physical adsorption does not apply in the cases discussed below. Exchange diffusion was preferred, as the cells appeared completely impermeable to phosphate in the cold, but freely permeable when the osmotic barrier was destroyed by acid or 5% (v/v) butanol in water. Some substances inhibited exchange at concentrations too low for stoichiometric displacement of adsorbed phosphate, but this does not disprove intracellular fixation, as it only indicates that a catalytic rather than a physical adsorption fixation mechanism is operating. More recent evidence is available to indicate * Present address : Virus Culture Laboratory, M.R.C. Laboratories, Carshalton, Surrey.

journal of general microbiology/journal of general microbiology

An Osmotic Barrier for Inorganic Phosphate in Chick Embryo Cells and its Stability during the Latent and Release Periods of Infection by Vesicular Stomatitis Virus