Oil-bodies as substrates for lipolytic enzymes

Plant seeds store triacylglycerols (TAGs) in intracellular organelles calle d oil-bodies or oleosomes, which consist of oil droplets covered by a coat of phospholipids and proteins. During seed germination, the TAGs of oil-bod ies hydrolysed by lipases sustain the growth of the seedlings. The mechanis m whereby lipases gain access to their substrate in these organelles is lar gely unknown. One of the questions that arises is whether the protein/phosp holipid coat of oil-bodies prevents the access of lipase to the oil core. W e have investigated the susceptibility of almond oil-bodies to in vitro lip olysis by various purified lipases with a broad range of biochemical proper ties. We have found that all the enzymes assayed were capable of releasing on their own free fatty acids from the TAG of oil-bodies. Depending on the lipase, the specific activity measured on oil-bodies using the pH-stat tech nique was found to range from 18 to 38% of the specific activity measured o n almond oil emulsified by gum arabic. Some of these lipases are known to h ave a dual lipase/phospholipase activity. However, no correlation was found to exist between the ability of a lipase to readily and efficiently hydrol yse the TAG content of oil-bodies and the presence of a phospholipase activ ity. Kinetic studies indicate that oil-bodies behave as a substrate as othe r proteolipid organelles such as milk fat globules. Finally we have shown t hat a purified water-soluble plant lipase on its own can easily hydrolyse o il-bodies in vitro. Our results suggest that the lipolysis of oil-bodies in seedlings might occur without any pre-hydrolysis of the protein coat. (C) 2001 Elsevier Science B.V. All rights reserved.