The photolysis of metarhodopsin I, produced from cattle rhodopsin, has been investigated. The rate of conversion of meta I to rhodopsin has little, if any, "dark" component i.e. rhodopsin appears nearly as fast as the meta I absorbs light. This differs from what was found in the studies on meta II where an easily measured dark component exists. Also different from the results on meta II was the fact that no product, analogous to P470, was noticed when meta I was flashed. The extent of photoreversibility of meta I to rhodopsin was also investigated and was found to depend both on pH and temperature. The reversibility was nearly abolished at pH 4 and reached an upper limit at pH 6.5 or above. The reversibility was decreased with increasing temperature and these results were interpreted in terms of a mechanism recently proposed by Williams (1970): MI ← MI' ← MII + H+. Assuming meta I-prime to be photoirreversible, the two equilibrium constants were measured over the range: 1-18°. The thermodynamic parameters were calculated and it was found that ΔH for the MI-MI' reaction is 11.3 kcal/mole and for the MI'-MII reaction it is 1.8 kcal/mole. The sum of these, 13.1 kcal/mole, is exactly the value obtained by Matthews et al. (1963) for the overall reaction: MI-MII. © 1971.
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