The process of lipolysis, or breakdown of fats. There are a range of receptors on the membrane, or surface of the fat cell, which respond in a number of different ways to a range of messenger chemicals, or hormones. These are a bit like locks and keys which open the cell up for a cascade of events which then result in the enzyme HSL being activated to break down TG into FFA plus glycerol for release into the bloodstream. This metabolic cascade, or amplification of chemical reactions, is common in nature and enables a much larger response to a small stimulus to occur. Enzymes, or catalysts, also take part in these reactions enabling them to be sped up to the rate necessary for a biological response.
There are over 30 different receptors, or ‘locks’ which have been identified to date on the fat cell membrane, and many more likely to be discovered. The ? (beta) receptors are important receptors which facilitate lipolysis. The hormones which lock into the 8 receptors are adrenalin and noradrenalin (catecholamines), both of which are involved in responses to stress. Factors affecting catecholamine production (and therefore potentially lipolysis), include exercise, cold, stress, caffeine and some other stimulant drugs, such as the amphetamines (which have, in the past, been the basis for some weight control drugs).
The lypolytic hormones ‘unlock’ a cell receptor, and the signal is then amplified through proteins inside the cell which in turn generate a substance called cyclic AMP (cAMP) which continues a cascade of events ending in the activation of the enzyme hormone sensitive lipase (HSL). This is the final step in the enzymatic attack on stored triglycerides which are hydrolysed (broken down) into FFA and glycerol. The circulating FFAs released may then be re-esterified back into the fat cell, or released to the circulation or taken up by other tissues to produce energy. The final combustion of FFAs in cells to produce energy is known as fat oxidation, and is the process by which stored body fat is ultimately used.
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