Sunday, October 2, 2011

Fat Tissue Regulation: Part IV ~ How Acylation Stimulating Protein Works

Lipases are a tricky bunch of enzymes when one is looking to the action of an enzyme to extrapolate to overall regulation of fat mass.    What do lipases do?  They are enzymes that facilitate lipolysis, which is the breaking apart of triglycerides into glycerol and three fatty acids.  We have digestive lipases that break dietary triglycerides down so that they can be absorbed, but once absorbed they are packaged again back into triglycerides for transport to cells.  These triglycerides are packaged in chylomicrons.  There are lipases both in circulation and attached to all of our different cells, called lipoprotein lipases (LPL's) that break down triglycerides to free up fatty acids.  Those associated with the cells are doing so to facilitate uptake of the fatty acids.  Here's where it gets tricky, because lipases associated with, say, muscle cells, are acting to bring fatty acids into the cell to be oxidized for energy.  But the lipases associated with fat cells?  These are acting to bring fatty acids into the cells for the purposes of being re-esterified into triglyceride stores.  Then there are lipases within cells, like hormone sensitive lipase, HSL, in fat cells.  The function of this lipase is "mobilization" of fat stores -- breaking down triglycerides to release fatty acids.

If you're locked into a "fat burning" paradigm then, you want high HSL and LPL activity in fat cells and non-fat tissue cells respectively, and low adipocyte LPL activity.  Fats can't get into cells without lipolysis.  But this isn't how our human metabolism works.
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