<P> In 1995, researchers identified the glyoxylate cycle in nematodes . In addition, the glyoxylate enzymes malate synthase and isocitrate lyase have been found in animal tissues . Genes coding for malate synthase have been identified in other metazoans including arthropods, echinoderms, and even some vertebrates . Mammals found to possess these genes include monotremes (platypus) and marsupials (opossum) but not placental mammals . Genes for isocitrate lyase are found only in nematodes, in which, it is apparent, they originated in horizontal gene transfer from bacteria . </P> <P> The existence of glyoxylate cycles in humans has not been established, and it is widely held that fatty acids cannot be converted to glucose in humans directly . However, carbon - 14 has been shown to end up in glucose when it is supplied in fatty acids . Despite these findings, it is considered unlikely that the 2 - carbon acetyl - CoA derived from the oxidation of fatty acids would produce a net yield of glucose via the citric acid cycle--however, acetyl - CoA can be converted into pyruvate and lactate through the ketogenic pathway . Put simply, acetic acid (in the form of acetyl - CoA) is used to partially produce glucose; acetyl groups can only form part of the glucose molecules (not the 5th carbon atom) and require extra substrates (such as pyruvate) in order to form the rest of the glucose molecule . But a roundabout pathway does lead from acetyl - coA to pyruvate, via acetoacetate, acetone, hydroxyacetone (acetol) and then either propylene glycol or methylglyoxal . </P> <P> In mammals, gluconeogenesis has been believed to be restricted to the liver, the kidney, the intestine, and muscle, but recent evidence indicates gluconeogenesis occurring in astrocytes of the brain . These organs use somewhat different gluconeogenic precursors . The liver preferentially uses lactate, alanine and glycerol (especially alanine) while the kidney preferentially uses lactate, glutamine and glycerol (especially glutamine). Lactate from the Cori cycle is quantitatively the largest source of substrate for gluconeogenesis, especially for the kidney . The liver uses both glycogenolysis and gluconeogenesis to produce glucose, whereas the kidney only uses gluconeogenesis . After a meal, the liver shifts to glycogen synthesis, whereas the kidney increases gluconeogenesis . The intestine uses mostly glutamine and glycerol . </P> <P> Propionate is the principal substrate for gluconeogenesis in the ruminant liver, and the ruminant liver may make increased use of gluconeogenic amino acids, e.g. alanine, when glucose demand is increased . The capacity of liver cells to use lactate for gluconeogenesis declines from the preruminant stage to the ruminant stage in calves and lambs . In sheep kidney tissue, very high rates of gluconeogenesis from propionate have been observed . </P>

Where does gluconeogenesis take place in eukaryotic cells