Acetoacetate formation

Acetoacetate formation has previously been carried out by using the... 

The mechanism of acetoacetate formation was then studied by isotopic methods. At first, experiments by Weinhouse and collaborators using liver slices seemed to indicate a random recombination of 2-carbon fragments to result in acetoacetate formation. Octanoate labeled with C in the carboxyl group was found to produce acetoacetate with almost equal label in the carboxyl and carbonyl groups. Later work with liver... 

It has been found that isovalerate-3-C yields acetoacetate labeled primarily in the carbonyl position and isovalerate 4,4 -C -yields acetoacetate with radioactivity concentrated in the methylene and methyl carbons, and approximately to the same extent in each portion. This shows that all 3 carbons of the isopropyl group are involved in acetoacetate formation, and it eliminates the possibility that a highly acetylating 2-carbon fragment is formed from the isopropyl group. The origin of the carboxyl carbon of the acetoacetate by the reversible fixation of CO2 by acetone to yield acetoacetate has been established by a number of investigators. ... 

The catabolism of glycine by way of serine and pyruvic acid is probably of little quantitative significance. This is indicated by the low degree of acetoacetate formation from glycine and serine in liver slices in contrast to the formation of acetoacetate from lactate SO). Furthermore, it is probable that serine serves as a source of glycine in the organism rather than the reverse. This is discussed under serine biosynthesis in Chapter 15. 

Carboxyl-labeling yielded acetoacetate with tracer in the carboxyl and carbonyl carbons. This shows that all three carbons of the isopropyl group are involved in acetoacetate formation, and it eliminates the possibility that a highly acetylating two-carbon fragment is formed from the isopropyl group. 

IV. Theories of Fatty Acid Breakdown and Acetoacetate Formation. 285... 

IV. THEORIES OF FATTY ACID BREAKDOWN AND ACETOACETATE FORMATION... 

The early theories of fatty acid breakdown and acetoacetate formation had three points in common (1) one or more alternate carbons of a fatty acid, starting with the carboxyl carbon, became oxidized (2) acetoacetate was always derived intact from the terminal four carbons and (3) 2-carbon fragments, if formed, did not condense to form acetoacetate. Much speculation did, however, center around this latter point. 

Although nearly all of the studies of acetoacetate formation from isotopic fatty acids have been carried out in vitro, there is no doubt that the formation of the ketone body from 2-carbon fragments occurs in the intact animal. Hexanoate-l-C, when injected intravenously into the goat, forms acetoacetate with isotope in the acetone moiety (presumably only in the carbonyl carbon) and in the carboxyl carbon. This provides direct evidence for the formation of 2-carbon fragments from fatty acids by the intact animal. 

Effect of the Rate of Acetoacetate Formation. The rate of acetoacetate formation apparently influences the C 0 C OOH ratio in cases where the conversion of fatty acid-2-carbon fragments to acetoacetate is not already... 

Fig. 5A. R vs. Ri for fatty acids of various chain lengths. The figure applies only to the special case of acetoacetate formation where 2-carbon fragments are converted quantitatively to acetoacetate (see text). R = C 0 C 00H ratio for carboxyl-labeled fatty acids Ri = C 0 C 00H ratio for penultimately labeled fatty acids c = number of carbons in a fatty acid (even series) / fractional degree of the conversion, (CH3CO—) —> (—CH2CO—) / = 1.0 is represented by the point (R = 1.0, Ri = 1.0). Note that when R = 1.0, Ri > 1.0. Small rectangular area is enlarged in Fig. 5B.

The third and fourth possible sites may be excluded by the present results that acetoacetate formation from octanoic acid (lFig.7) and COa formation from l- C-octanoic acid ( Fig.6 ) were not affected by pantethine under the conditions where palmitate oxidation was stimulated by pantethine. If either of these two oxidation cycles could be affected by pantethine, octanoate oxidation in either system had to be stimulated, because octanoic acid is non-enzymatically permeable through the mitochondrial membranes. 

Another type of CO2 fixation which has been observed in photosynthetic tissue concerns Siegel s observation of acetoacetate formation from acetone by purple bacteria (315). The energy for this... 

Oxidation of Fatty Acids.— The molecule of an unsaturated fatty acid such as oleic, is most likely to undergo oxidative attiack at the point of unsaturation the saturated acids, stearic and palmitic, are degraded by terminal oxidation. Fat oxidation occurs chiefly, if not entirely, in the liver, and under normal conditions the process is complete, and ends in COg and HjO. However, in diabetes and other conditions of carbohydrate inadequacy, fat oxidation in the liver is unable to proceed beyond acetoacetic acid, which suggests that this compound is an intermediate in fat metabolism. The natural fatty acids almost without exception contain an even total number of carbon atoms, and to explain the process of acetoacetic formation, Knoop proposed, in 1904, his theory of /5-oxidation of the fatty acids, according to which, the point of oxidative attack is the carbon atom in the /5-position, or next but one to the terminal carboxyl group. By this means the fatty acids are degraded two carbon atoms at a time. 

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