<P> However acetyl - CoA can be converted to acetoacetate, which can decarboxylate to acetone (either spontaneously, or by acetoacetate decarboxylase). It can then be further metabolized to isopropanol which is excreted in breath / urine, or by CYP2E1 into hydroxyacetone (acetol). Acetol can be converted to propylene glycol . This converts to formate and acetate (the latter converting to glucose), or pyruvate (by two alternative enzymes), or propionaldehyde, or to L - lactaldehyde then L - lactate (the common lactate isomer). Another pathway turns acetol to methylglyoxal, then to pyruvate, or to D - lactaldehyde (via S-D - lactoyl - glutathione or otherwise) then D - lactate . D - lactate metabolism (to glucose) is slow or impaired in humans, so most of the D - lactate is excreted in the urine; thus D - lactate derived from acetone can contribute significantly to the metabolic acidosis associated with ketosis or isopropanol intoxication . L - Lactate can complete the net conversion of fatty acids into glucose . The first experiment to show conversion of acetone to glucose was carried out in 1951 . This, and further experiments used carbon isotopic labelling . Up to 11% of the glucose can be derived from acetone during starvation in humans . </P> <P> The glycerol released into the blood during the lipolysis of triglycerides in adipose tissue can only be taken up by the liver . Here it is converted into glycerol 3 - phosphate by the action of glycerol kinase which hydrolyzes one molecule of ATP per glycerol molecule which is phosphorylated . Glycerol 3 - phosphate is then oxidized to dihydroxyacetone phosphate, which is, in turn, converted into glyceraldehyde 3 - phosphate by the enzyme triose phosphate isomerase . From here the three carbon atoms of the original glycerol can be oxidized via glycolysis, or converted to glucose via gluconeogenesis . </P> <P> Fatty acids are an integral part of the phospholipids that make up the bulk of the plasma membranes, or cell membranes, of cells . These phospholipids can be cleaved into diacylglycerol (DAG) and inositol trisphosphate (IP) through hydrolysis of the phospholipid, phosphatidylinositol 4, 5 - bisphosphate (PIP), by the cell membrane bound enzyme phospholipase C (PLC). </P> <P> An example of a diacyl - glycerol shown on the right . This DAG is 1 - palmitoyl - 2 - oleoyl - glycerol, which contains side - chains derived from palmitic acid and oleic acid . Diacylglycerols can also have many other combinations of fatty acids attached at either the C - 1 and C - 2 positions or the C - 1 and C - 3 positions of the glycerol molecule . 1, 2 disubstituted glycerols are always chiral, 1, 3 disubstituted glycerols are chiral if the substituents are different from each other . </P>

When a cell uses fatty acid for aerobic respiration