Evidence for monovalent phosphate transport in Ehrlich ascites tumor cells

✍ Scribed by Jesse W. Bowen; Charles Levinson

• Publisher: John Wiley and Sons
• Year: 1983
• Tongue: English
• Weight: 744 KB
• Volume: 116
• Category: Article
• ISSN: 0021-9541
• DOI: 10.1002/jcp.1041160204

✦ Synopsis

In an effort to determine whether the Na+-dependent P, transport system of Ehrlich ascites tumor cells exhibits specificity for H,PO, or HPO, :, P, fluxes were determined by measuring "P,-P, self-exchange. Three experimental approaches were employed. First, the effect of pH on steady-state P, transport at 0.5 and 5 m M was studied. Second, the relationship between P, transport and P, concentration (0.25-9.2 mM) at pH 5.6 and 7.9 was determined. Third, the dependence of P, transport on [H,PO,-I (0.05-4.2 mM) at constant [HPO;,] (0.5 mM), and the converse, [HPO,-'] (0.06-4.5 mM) at constant [HLPO, 1 (0.5 mM), was evaluated. K, (apparent half-saturation constant) and

(maximal transport rate) were calculated by two methods:

weighted linear regression (WLR) and a nonparametric procedure. The dependence of P, flux on pH indicates that optimum transport occurs at pH 6.9. P, transport decreases as pH i s reduced when extracellular P, is either 0.5 or 5 mM. However, at pH 7.9, P, flux i s reduced only in 0.5 m M P,. At pH 5.6, H>PO,-comprises 93% of the total P, present, and the calculated K, is 0.055 f 0.026 m M (WLR). This is the same as the K, determined from the initial phase of the flux vs. [H,PO,-] relationship (0.056 * 0.020 mM). However, at pH 7.9 (where 94% of P, is HPO, >), the measured K, is 0.58 f 0.11 m M (WLR), which i s ten times higher than at pH 5.6. This value is also five times greater than the K, calculated from the flux vs. [HPO,-'] curve (0.106 f 0.16 mM). Kinetic parameters calculated by the nonparametric method, though somewhat different, gave similar relative results. Taken together, these results support two conclusions: (1) H,PO, is the substrate for the Na+-dependent P, transport system of the Ehrlich cell, and (2) Hcan inhibit P, transport.