Acyl-(acyl-carrier protein) hydrolase from squash cotyledons specific to long-chain fatty acids: purification and characterization.

Acyl-(acyl-carrier-protein) hydrolase (EC 3.1.2.14) releases fatty acids from the end-product of fatty acid synthesis in plastids for the subsequent synthesis of glycerolipids in the cytoplasm. Isoelectric focusing of chloroplast stroma proteins from squash cotyledons suggested that there were at least three isomeric forms of acyl-(acyl-carrier-protein) hydrolase having pI values of 4.5, 5.3 and 7.8. The pI 4.5 and pI 5.3 forms showed maximum activity at pH 9.8 whereas the activity of the pI 7.8 form increased within the range 6.2 to 10.2 but no optimum was seen. The pI 4.5 form was purified 100,000-fold from squash cotyledons. The highly purified fraction contained two polypeptides, whose molecular masses were estimated to be 35 kDa and 33 kDa by SDS-PAGE. It is suggested that the 33 kDa polypeptide was a degradation product of the 35 kDa polypeptide. Oleoyl-(acyl-carrier protein) was the preferred substrate of this enzyme over palmitoyl- and stearoyl-(acyl-carrier protein), whereas lauroyl-(acyl-carrier protein) was nearly inactive. These results indicate the enzyme is specific for long-chain acyl-(acyl-carrier protein).