Effects of photo-oxidizing analogs of fluorescein on the sarcoplasmic reticulum Ca2+-ATPase. Functional consequences for substrate hydrolysis and effects on the partial reactions of the hydrolytic cycle.

Erythrosin B was used to photo-oxidize the sarcoplasmic reticulum Ca2+-ATPase. The ATPase activity is rapidly and irreversibly inhibited by photo-oxidation with erythrosin. This inhibition is protected by the presence of ATP during the photo-oxidation period. After photo-oxidation, the steady-state phosphorylation by ATP remains almost unchanged, whereas phosphorylation by inorganic phosphate is impaired. The pseudo-first order rate constants for phosphorylation by 15 microM ATP at 25 degrees C are strongly inhibited when starting from either a Ca2+-bound or a Ca2+-free enzyme form, decreasing from 145 to 23 s-1 for the Ca2+-bound form and from 50 to 18 s-1 for the Ca2+-free form. Concurrently, the rate constants for dephosphorylation are also severely inhibited, changing from a fast double exponential to a very slow single exponential decay in the reverse direction and from a moderately slow single to a very slow single exponential decay in the forward direction. Ca2+ binding data show that the phosphorylated intermediate formed by the photo-oxidized enzyme contains two occluded Ca2+, and TNP-ATP fluorescence measurements indicate that it accumulates in a E1-P.Ca2-like conformation. Protection by ADP against glutaraldehyde-induced cross-linking indicates that ADP binding to Ca2+-ATPase is not impaired by photo-oxidation nor by free erythrosin. These data support the view that an ADP-insensitive, Ca2+-bound, slowly interconverting phosphoenzyme is formed. Thus, photo-oxidation with erythrosin B leads to impairment of phosphoryl transfer reactions and related conformational changes.