Abstract:Der Abbau und die Funktion der Citronensäure waren bis vor kurzem dunkel, da der Neef und K ei ehe l 1 ) aufgezeigte Abbau zu Acetondicarbonsäure (/2-Ketoglutarsäure) nur ein untergeordneter Nebenabbau der Citronensäure sein kann, weil reine Acetondicarbonsäure im Atmungsversuch nicht als Donator wirkt [Wagner-Jauregg 2 )].Erst vor kurzem ist durch neue Arbeiten von Knoop und Martius 3 ' 4 ' 5 ) gezeigt worden, daß der Abbau der Citronensäure in der Hauptsache nicht über die /?-Ketoglutarsäure, sondern über di… Show more
“…Knoop and Martins (38,48) and Martins (46) have shown that citric acid may arise through the condensation of pyruvic with oxaloacetic acid and subsequent oxidation, and that citric acid can be converted mto a;-ketoglutaric acid by an enzyme system in liver tissue, the intermediates probably being the well-known constituents of certain plants, aconitic and isocitric acids. Krebs (42) has confirmed this observation, as has also Breusch (18). The respiration of liver tissue was found to be stimulated to almost exactly the same extent by citric acid, cisaconitic acid, isocitric acid, and a-ketoglutaric acid.…”
Section: The Carbohydrate-organic Acid Metabolismmentioning
source of the carbon compound with which the ammonia combines, although this is a matter of debate at the present time. This view of the function of the amides has been shown in this laboratory to agree in general with observations on the tobacco (85) and beet (81) plants. The German investigators, however, believe that in acid plants, particularly in Begonia., Oxalis, and Rheum species, ammonia is chiefly dealt with by direct neutralization by an organic acid. They quote experimental results that show concentrations of ammonia nitrogen in the rhubarb petiole far higher than are usually encountered in other species, and call attention to the parallel production of organic acids and ammonia during growth as an evidence of neutralization as the mechanism for deahng with these large amounts of ammonia. Whether or not rhubarb possesses an amide synthesizing mechanism of the usual type is left uncertain by their experiments, but Kultzscher (44) maintains that amide synthesis plays a very small part in acid plants and distinguishes between ammonium ions and free ammonia. He makes use of an idea that probably originated with Prianischnikow (64) according to which only free ammonia is thought to be toxic and, thus, in the acid plant where, at high hydrogen ion activities, the ammonia tension is reduced to infinitesimal proportions, there is no necessity for conversion into a neutral amide. These observations induced us to examine the organic acids of the rhubarb plant in some detail, and it was found that, in a series of samples of leaves collected at different times in the season, there was no evidence of correlation between the changes in the quantities of ammonia and of any of the three chief organic acids, /-malic, oxalic, and citric acids (66). Connecticut Experiment Station Bulletin 424 ured volume of water. The second lot of samples consisted of 10 leaves each and received the key letter E, to signify that the tissue was to be extracted with water. The management of this series of samples was the same as the D series. The material assembled for analysis at the end of the experiment consisted of the following: Fresh leaf control: Blade tissue and petiole tissue in duplicate from 20 leaves (FDl, FD2); dried. Blade tissue and petiole tissue extracts in duplicate from 10 leaves (FE1,FE2). Residues from extraction of blade tissue and petiole tissue, dried.
“…Knoop and Martins (38,48) and Martins (46) have shown that citric acid may arise through the condensation of pyruvic with oxaloacetic acid and subsequent oxidation, and that citric acid can be converted mto a;-ketoglutaric acid by an enzyme system in liver tissue, the intermediates probably being the well-known constituents of certain plants, aconitic and isocitric acids. Krebs (42) has confirmed this observation, as has also Breusch (18). The respiration of liver tissue was found to be stimulated to almost exactly the same extent by citric acid, cisaconitic acid, isocitric acid, and a-ketoglutaric acid.…”
Section: The Carbohydrate-organic Acid Metabolismmentioning
source of the carbon compound with which the ammonia combines, although this is a matter of debate at the present time. This view of the function of the amides has been shown in this laboratory to agree in general with observations on the tobacco (85) and beet (81) plants. The German investigators, however, believe that in acid plants, particularly in Begonia., Oxalis, and Rheum species, ammonia is chiefly dealt with by direct neutralization by an organic acid. They quote experimental results that show concentrations of ammonia nitrogen in the rhubarb petiole far higher than are usually encountered in other species, and call attention to the parallel production of organic acids and ammonia during growth as an evidence of neutralization as the mechanism for deahng with these large amounts of ammonia. Whether or not rhubarb possesses an amide synthesizing mechanism of the usual type is left uncertain by their experiments, but Kultzscher (44) maintains that amide synthesis plays a very small part in acid plants and distinguishes between ammonium ions and free ammonia. He makes use of an idea that probably originated with Prianischnikow (64) according to which only free ammonia is thought to be toxic and, thus, in the acid plant where, at high hydrogen ion activities, the ammonia tension is reduced to infinitesimal proportions, there is no necessity for conversion into a neutral amide. These observations induced us to examine the organic acids of the rhubarb plant in some detail, and it was found that, in a series of samples of leaves collected at different times in the season, there was no evidence of correlation between the changes in the quantities of ammonia and of any of the three chief organic acids, /-malic, oxalic, and citric acids (66). Connecticut Experiment Station Bulletin 424 ured volume of water. The second lot of samples consisted of 10 leaves each and received the key letter E, to signify that the tissue was to be extracted with water. The management of this series of samples was the same as the D series. The material assembled for analysis at the end of the experiment consisted of the following: Fresh leaf control: Blade tissue and petiole tissue in duplicate from 20 leaves (FDl, FD2); dried. Blade tissue and petiole tissue extracts in duplicate from 10 leaves (FE1,FE2). Residues from extraction of blade tissue and petiole tissue, dried.
“…Ironically, Krebs himself was proving his theory on the cycle by fighting opposing views of other researchers who ignored the reversibility of several steps. Below is an excerpt from a reply to the criticisms of F. L. Breusch and of J. Thomas (Breusch, ; Thomas, ): Several of the criticisms arise solely from a misinterpretation of the theory. Thomas as well as Breusch draw incorrect conclusions from the theory and argue against the theory when these conclusions are not confirmed by their experiments.…”
Section: A Naïve Look At the Citric Acid Cyclementioning
The citric acid cycle forms a major metabolic hub and as such it is involved in many disease states implicating energetic imbalance. In spite of the fact that it is being branded as a 'cycle', during hypoxia, when the electron transport chain does not oxidize reducing equivalents, segments of this metabolic pathway remain operational but exhibit opposing directionalities. This serves the purpose of harnessing high energy phosphates through matrix substrate-level phosphorylation in the absence of oxidative phosphorylation. In this mini-review these segments are appraised, pointing to the critical importance of the alpha-ketoglutarate dehydrogenase complex dictating their directionalities.
“…This was the most convincing demonstration yet that the isomerization of the tricarboxylic acidsscitrate, cisaconitate, and isocitrateswas catalyzed by an enzyme different from that which carried out the oxidative decarboxylation of isocitrate to R-ketoglutarate. The name aconitase was proposed 2 and generally accepted for this enzyme (citrate (isocitrate) hydro-lyase, EC 4.2.1.3). The reactions catalyzed by aconitase are shown in Scheme 1.…”
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