Aconitase activity of IRP1 in LY cells. Aconitase activity of IRP1 was determined spectrophotometrically by measuring the disappearance of cis-aconitate at 240 nm. Aconitase activity values are expressed as a mean ± SE. Results are obtained from 6 independent measurements made on cytosolic extracts prepared from cells from separate cultures. The results are significantly different,P < .01.