The acyl chains of diacylglycerols are then deesterified by the enzyme lipolytic acyl hydrolase, liberating free fatty acids. Unsaturated fatty acids with a cis-1.4 pentadiene structure (linoleic acid, linolenic acid) are acted upon by lipoxygenase, causing the peroxidation of fatty acids. This step may also cause the production of activated oxygen pecies such as singlet oxygen, superoxide, and peroxy radicals. The peroxidation product of linolenic acid can be 9-hydroperoxy linoleic acid or 13-hydroperoxy linoleic acid.