Activation of alkylthioacrylic acids in subcellular fractions of rat tissues: a new spectrophotometric method for assay of acyl-CoA synthetase.

Alkylthioacrylic acids are activated by the long-chain acyl-CoA synthetase in mitochondria and microsomes from rat liver, heart and kidney. The activation of a corresponding dicarboxylic acid was also demonstrated. The highest rate of activation was found in liver microsomes. The activation rate of the long-chain alkylthioacrylic acids is about one third of that of corresponding normal long-chain saturated fatty acids. The short-chain (methyl-, butyl-) thioacrylic acids showed no detectable activation in intact mitochondria or mitochondrial extracts. The cofactor requirement, chain length specificity and mutual inhibition of normal fatty acids and alkylthioacrylic acids in activation indicate that the long-chain fatty acid activating enzyme (long-chain acyl-CoA synthetase) of microsomes and mitochondria accepts alkylthioacrylic acids as substrates, while the short- and medium-chain fatty acid activating enzymes of the mitochondrial matrix do not. The UV absorption at 312 nm of the coenzyme A esters of alkylthioacrylic acid with a high molar extinction coefficient (22 mM-1 cm-1) makes a specific spectrophotometric assay of the long-chain acyl-CoA synthetase possible in spite of a slower reaction rate than with normal fatty acids.