Abstract
Lyophilized rod outer segments isolated from partially dark-adapted pig retinas possess ATPase activity. Magnesium stimulates activity, optimally when at about the same concentration as ATP. Maximum activity is obtained when both sodium and potassium also are present; optimal concentrations are about 15 m M Na + and 6 m M K +. Further increases in potassium concentration are inhibitory, but this inhibition can be partially overcome by raising the sodium concentration. In the presence of magnesium, sodium alone stimulates slightly; potassium alone has no effect. Ouabain inhibits activity to the level observed when only magnesium and sodium are present. Inhibition is half-maximal at 6.3 × 10 −7 M. Ca ++ and pCMB also inhibit; DNP has no effect. Adenosine triphosphate is hydrolyzed more than twice as rapidly as eight other phosphate esters tested, both in the presence and absence of ouabain. For the following reasons we conclude that the ATPase activity of the rod outer segments is not a property of the visual pigment, as has been suggested: (i) The rate of hydrolysis of ATP by dark-adapted rods is the same in light as it is in darkness, whether or not hydroxylamine or ouabain is present. (ii) The ATPase activity of pig retinas, exclusive of rod outer segments, is roughly the same, per unit dry weight, as that of the outer segments alone. (iii) By treatment of dark-adapted outer segments with digitonin it is possible to effect a partial separation of rhodopsin and ATPase activity. (iv) CTAC extracts rhodopsin from outer segments but destroys all the ATPase activity in both the extract and the residue.
Published Version
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