The alkaline phosphatase prepared from kidneys of domestic chicks is a tetramer (Mr 270,000) consisting of identical subunits (Mr 68,000). The tetramer may be dissociated by detergent treatment to a dimer (M(r) 150,000) with no loss of catalytic activity. The tetramer probably represents the in vivo state. The enzyme is stimulated by Mg^2+ and inhibited noncompetitively by Zn^2+ and levamisole. The stimulation and inhibition show similar pH dependencies. Evidence for an essential histidine is provided by sensitivity of the enzyme to diethyl pyrocarbonate. It is suggested that chick kidney and bone phosphatases could be expressed by different genes.

1. Proximal-tubule cells isolated from mouse kidney after digestion with collagenase take up Pi by an Na+-dependent and saturable process mediated by the Na+-Pi co-transporter of the brush-border membrane. 2. Pi depletion of the cells is accompanied by a stimulation of Pi-transport activity. Kinetic investigations reveal that Vmax. is increased by 90% and Km decreased by 50% after Pi depletion. Transport activity returns to normal values after incubation for 30 min at 37 degrees C of Pi-depleted cells in normal medium containing 1 mM-Pi, but the fall in transport activity under these conditions is inhibited by colchicine. 3. The energy of activation of Na+-Pi co-transport activity of depleted cells differs greatly from that found for normal replete cells. 4. The results provide evidence that stimulation of transport by Pi depletion arises from an increase in the number of carrier sites in the brush-border membrane. Additionally, changes in the properties of the transporter occur which may reflect altered phospholipid-carrier-protein interaction in the Pi-depleted condition.

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