Effect of pressure and calcium on the reversible inhibition of the sarcoplasmic‐reticulum calcium‐transport enzyme and on its tryptic cleavage pattern

The reversible inhibition of the sarcoplasmic‐reticulum calcium‐transport enzyme by pressure at room temperature is accompanied by a significant enhancement of the accessibility of the enzyme to tryptic cleavage dependent on the presence of calcium. The calcium‐transport enzyme activity was monitored with dinitrophenyl phosphate as substrate. Pressure in the range 0.1 – 100.0 MPa affects trypsin cleavage of the control substrate N‐α‐benzoyl‐L‐arginine‐4‐nitroanilide hydrochloride little in the presence and absence of calcium. In contrast, application of 100.0 MPa to the calcium‐transport enzyme at room temperature accelerates subsequent tryptic cleavage at the T2 but not at the T1 cleavage site [C. J. Brandl et al. (1986) Cell 44, 597–607]. Pressure application during tryptic digestion likewise solely affects cleavage at T2 which proceeds slowly in the absence but rapidly in the presence of calcium. At atmospheric pressure in the absence of calcium and at high pressure in the absence and presence of calcium new cleavage sites are exposed giving rise to new subfragments B1–3 in addition to the established peptides A1 and A2. Under pressure and in the presence of calcium, A1 and A2 rapidly disappear indicating the presence of calcium‐binding sites in these peptides. In contrast, the B1–3 peptides which are most likely derivates of the B fragment accumulate in the presence and absence of calcium. In contrast to tryptic cleavage at atmospheric pressure, tryptic cleavage of the A as well as of the B fragment tends to completion under pressure. In parallel to the disappearance of the A and B fragments calcium‐dependent substrate hydrolysis vanishes. Computation of activation volumes for pressure‐induced reversible enzyme inhibition and for tryptic cleavage furnished closely related volumes of opposite signs of 20–40 ml/mol and 80–100 ml/mol in the ranges 0.1–40.0 MPa and 40.0–100.0 MPa, respectively. Thus pressure produces reversible changes in the calcium‐transport enzyme which activates and modifies tryptic‐cleavage patterns at the T2 site of the A segment and at sites in its subfragments in the presence of calcium, i.e. if the enzyme resides in its E1 state. In contrast tryptic cleavage of the B fragment is accelerated by pressure independently of the presence of calcium.