Polyribosomes from Peas: III. Stimulation of Polysome Degradation by Exogenous and Endogenous Calcium

Attempts were made to isolate microsomes from Pisum sativum L. var. Alaska by low speed centrifugation of a postmitochondrial supernatant made 8 mm in Ca(2+). However, the addition of Ca(2+) in concentrations as low as 1 mm to the postmitochondrial supernatant resulted in extensive polysome degradation. Degradation was dependent on both Ca(2+) concentration and the duration of incubation. Resuspension of isolated polysomes in Ca(2+)-containing buffer did not result in degradation, whereas resuspension in Ca(2+)-containing postpolysomal supernatant did. Both Ca(2+) and a heat-labile factor in the supernatant were required for polysome degradation. The degradation in the homogenate with or without added Ca(2+) could be reduced by (a) dilution with larger volumes of grinding buffer, (b) increasing the concentration of tris-HCl in the grinding buffer, (c) adding diethylpyrocarbonate or ethyleneglycol-bis (2-aminoethylether) tetraacetic acid (a specific calcium chelator) prior to homogenization or immediately after the addition of Ca(2+). Endogenous Ca(2+) can increase the destruction of polysomes during their isolation in this tissue, presumably by activating a ribonuclease. Addition of Ca(2+) is not a useful technique for separating undegraded free and membrane-bound polyribosomes.