Detergent‐solubilization, purification, and characterization of membrane‐bound 3‐hydroxy‐3‐methylglutaryl‐coenzyme A reductase from radish seedlings

3‐Hydroxy‐3‐methylglutaryl‐CoA reductase (NADPH) was solubilized with polyoxyethylene ether (Brij) W‐1 from a heavy‐membrane fraction, sedimented at 16000x g from a cell‐free homogenate of four‐day‐old, dark‐grown radish seedlings ( Raphanus sativus L.). Approximately 350‐fold purification of the solubilized enzyme activity was achieved by (NH 4 ) 2 SO 4 precipitation followed by column chromatography on DEAE‐Sephadex A‐50, blue‐dextran‐agarose and HMG‐CoA‐hexane‐agarose. The presence of detergent, which was required at all times to maintain activity, did not interfere with the chromatographic procedures used. Sucrose density centrifugation suggested an apparent molecular mass of 180 kDa with subunits of 45 kDa (polyacrylamide gel electrophoresis in the presence of sodium dodecylsulphate). The enzyme was stable at 67.5°C for 30 min in the presence of glycerol, dithioerythritol and detergent. Studies of enzyme stability and activation indicate that the enzyme is a hydrophobic protein with free thiol groups that are essential for full activity. The activation energy was estimated to be 92 kJ (Arrhenius plot). Antibodies raised against rat liver and yeast hydroxymethylglutaryl‐CoA (HMG‐CoA) reductase failed to bind or inactivate the radish enzyme. When both HMG‐CoA and NADPH concentrations were varied, intersecting patterns were obtained with double‐reciprocal plots. The apparent K m values determined in this way are 1.5 μM [( S )‐HMG‐CoA], and 27 μM (NADPH). Concentrations of NADPH greater than 150 μM caused substrate inhibition at low HMG‐CoA concentrations resulting in deviations from linearity in secondary plots. Analysis of these data and the product inhibition pattern suggest a sequential mechanism for the reduction of HMG‐CoA to mevalonic acid with HMG‐CoA being the first substrate binding to the enzyme, followed by NADPH.