= Pigeon breast muscle evidence for a metabolic cycle
<Hans Adolf Krebs> described the experimental reasoning in his 1953 Nobel lecture. Pigeon breast <skeletal muscle> retained strong oxidative activity after preparation in a Latapie mincing mill and suspension in aqueous medium. Small <fumarate> additions increased <oxygen> uptake by more than complete <oxidation> of the added <fumarate> could explain; added four-<carbon> acids could still be detected afterward. His Table 1 compares the increase in uptake with that calculated from complete consumption of the addition. These comparisons challenge the interpretation that the addition merely supplied extra fuel.<The citric acid cycle by Hans Adolf Krebs>[The citric acid cycle, Nobel Lecture, 11 December 1953, printed pp. 400–401 and Table 1: minced-muscle preparation, uptake comparison and retained acid; authored transcript also available at https://studyres.com/doc/1333386/hans-a.-krebs---nobel-lecture]{ref}
The lecture identifies the preparation apparatus but does not describe the gas-measuring instrument behind Table 1. The result supports regeneration, not a unique complete <reaction mechanism>. Its later discussion combines <citrate> formation from <oxaloacetate> with breakdown back toward the four-<carbon> intermediates to close the cycle. <Competitive inhibition> supplies another comparison: <malonate> inhibits <succinate dehydrogenase>, and <succinate> accumulates. Such evidence links a blocked step to the surrounding <metabolic pathway>; damaged tissue and restricted inhibitor access can complicate interpretation.<The citric acid cycle by Hans Adolf Krebs>[The citric acid cycle, printed pp. 401–404, Figures 1–2 and malonate discussion: cyclic interpretation and inhibitor-access limitation; p. 399, preparation damage]{ref}
An increase above the added fuel's own <oxygen> demand is a particularly revealing control on the explanation: it compares the proposed cause with the largest effect that simple consumption could produce. The modern <mitochondrial oxygen electrode assay> asks a different question about a prepared <mitochondrion>, testing <respiratory control> through <ADP> additions. Neither an <oxygen> uptake signal nor an accumulating intermediate is by itself a direct measurement of <ATP> production.
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