The Heparin Clearing Reaction

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Abstract
Clearing factor lipase is rapidly released into the circulating blood following the intravenous injection of small amounts of heparin. Its site of action in the body in the absence of exogenous heparin is unknown, although the available evidence suggests that it may be at the walls of the blood vessels. Lipases must be involved in the utilization of both dietary and depot fat since triglyceride fatty acids cannot be oxidized directly in the tissues. The evidence which suggests that the clearing factor lipase may have a specific function in facilitating transport of dietary fat to the tissues has been reviewed here. Stored triglyceride may be hydrolysed within the cells of the fat depots prior to the mobilization of depot fat. However, there is at present no evidence that the clearing factor lipase is responsible for such hydrolysis. An alternative possibility, that lipolysis occurs subsequent to the mobilization of the depot triglyceride, might, however, involve the action of this enzyme. Mobilized triglyceride would presumably be carried in the blood in low density lipoprotein complexes, and triglyceride in this form has been shown to be an adequate substrate for the clearing factor lipase. The suggestion that the clearing factor lipase may have a general function in facilitating the transport of triglyceride fatty acids out of the blood is admittedly a speculative one. Whether or not it is supported by further work, however, the interesting properties of the clearing factor lipase, together with its general interest in relation to the problem of fat transport, fully justify our continued interest in it.