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Ketone and lactate Metabolism: An exchange of conclusions
CORRESPONDENCE
260
KETONE AND LACTATE
METABOLISM:
To the Editor: In the May issue of Metabolism, Landau and Wahren wrote an editorial’ criticizing the conclusions of our work assessing the appearance rate of new ketone bodies across the forearm.’ We were surprised by the repetitious critique, since Dr Landau had long ago expressed his opinions about the potential effects of isotopic exchange on our results3 and since both his group4 and ours had subsequently published primary, albeit conflicting, data on this issue. Since the editorial in question, the full details of our own work5 have appeared.h In his editorial. Dr Landau once again3 described the biochemical capacity for isotopic exchange between acetoacetyl-CoA and labeled acetoacetate in muscle. He further supports his argument with “physiological” data from one of his own publications in hepatectomized dogs4 We do not deny the biochemical capacity for isotopic exchange.h We do, however, question the appropriateness of the hepatectomized dog, an animal with low ketogenic capacity, to reflect the circumstance in intact humans possessing a functioning liver. The precise issue that we investigateds,h was whether the mechanism of isotopic exchange contributed significantly to the tracer-measured appearance rate of acetoacetate across human muscle tissue. In part, these investigations were based on the detection of such exchange by the positional rearrangement or randomization of labeled acetoacetate carbon transversing muscle tissue. Such rearrangement is dictated by the biochemical scenario described by Dr Landau. We were unable to detect any randomization of labeled carbon. We concluded, therefore, that the exchange process did not contribute significantly to the results observed. In his editorial, Dr Landau minimizes our negative data by stating that “the isotopic exchanges cannot be equated with production or Surely, on the other hand, if we had shown such utilization.” exchange Dr Landau would have used it as support for his contention that isotopic exchange was the mechanism responsible for our thesis about the appearance of new ketone bodies across the human forearm. Apparently, the only data that will satisfy Dr Landau about the practical consequences of the theoretical potential for isotopic exchange are data that support his own position.
AN EXCHANGE
OF CONCLUSIONS
Further, to expand our understanding of this issue6 with experimental data rather than conjecture, we also infused normal adults with Intralipid plus heparin in order to raise the level of free fatty acid delivery to forearm muscle to a degree comparable to that seen in hyperglycemic diabetic adults with insulin insufficiency.h Via fatty acid oxidation, this maneuver should produce comparable intramuscular unlabeled acetoacetyl-CoA pool and, therefore, similar passive exchange dilution of labeled acetoacetate tracer across the muscle bed. Nonetheless, the degree of ketone body isotopic dilution across the forearm was vastly different between these groups. These data, while not unequivocal proof, support our own hypothesis but do not support Dr Landau’s contention. From these data, therefore, we concluded that simple isotopic exchange alone cannot fully account for the apparently increased rate of appearance of unlabeled ketone bodies measured across the forearm of diabetic individuals. In conclusion, while we appreciate continued discussion of the issues of isotopic exchange in tracer kinetics, we object to implications of the position espoused in the aforementioned Metabolism editorial, namely that we were unaware of the implications of such exchange on our results and that our data can be explained by exchange alone. In fact, our published experiment@ were designed to test the significance of the such exchange in human physiology and, in fact, provide data that support our position rather than Landau’s on this issue
Ramano Nosadini Department of Internal Medicine University of Padua, Italy Angelo Avogaro Department of Clinical Medicine University of Padua, Italy Dennis M. Bier Department of Pediatrics Washington University St Louis, MO
REFERENCES 1. Landau BR, Wahren J: Nonproductive exchanges: The use of isotopes gone astray. Metabolism 41:457, 1992 2. Nosadini R, Avogaro A, Sacca L. et al: Ketone body metabolism in normal and diabetic human skeletal muscle. Am J Physiol 249:E131-E136.1985 3. Landau BR: A potential pitfall in the use of isotopes to measure ketone body production. Metab Clin Exp. 35:94, 1986 4. Des Rosiers C, Montgomery JA, Garneau M. et al: Pseudoke-
togenesis in hepatectomized dogs. Am J Physiol 258:E519-E528, 1990 5. Avogaro A, Doria A, Valerio A, et al: Apparent ketone body “production” from the human forearm is not due to isotopic exchange via acetoacetyl-CoA thiolase. Diabetes 38:22A. 1989 (suppl2) 6. Avogaro A, Doria A, Gnudi L, et al: Forearm ketone body metabolism in normals, and insulin dependent diabetic patients. Am J Physiol263:E261-E267,1992
REPLY
necessary consequence of pseudoketogenesis is the transfer of label from [3,4-‘“CzJacetoacetate to form [1,2-“Czlacetoacetate, ie, randomization of labeled carbons, which they were unable to detect. Let me explain (1) why Avogaro et al’s interpretation of their data is erroneous, (2) why their data are explained by exchange alone, and (3) why their data confirm the existence of pseudoketogenesis with more occurring in muscles of diabetic subjects. First, please look at Fig 1 which shows the scheme used by Avogaro et al’ for their interpretation. Acetyl-CoA is not, as the scheme indicates. a necessary intermediate in the formation of acetoacetyl-CoA from fatty acids (FA). For Fig I to be correct, an arrow must be introduced from FA to acetoacetyl-CoA. That is so because the last four carbons of FA are directly converted to acetoacetyl-CoA6 (as depicted in references 5, 7, and 8 and shown in Fig 2). Thus, the biochemical scenario described by my col-
To the Editor: I welcome the opportunity to respond, with the concurrence of Drs Wahren and Brunengraber, to the comments directed toward me by Drs Nosadini, Avogaro, and Bier. now that full details of their work are available.’ Avogaro et alie investigated the metabolism of [3,4-r3Cs] acetoacetate in human forearm muscle. They observed net uptake of ketone bodies (KB) and dilution of i3C enrichment of KB across muscle, the extent of dilution varying with conditions. They interpret their data as evidence of KB production by muscle and more by diabetic muscle. They discount as an explanation pseudoof KB by isotopic ketogenesis, 4.s ie, dilution of the i3C enrichment exchange between acetoacetate and acetoacetyl-CoA. They claim a
260
KETONE AND LACTATE
METABOLISM:
To the Editor: In the May issue of Metabolism, Landau and Wahren wrote an editorial’ criticizing the conclusions of our work assessing the appearance rate of new ketone bodies across the forearm.’ We were surprised by the repetitious critique, since Dr Landau had long ago expressed his opinions about the potential effects of isotopic exchange on our results3 and since both his group4 and ours had subsequently published primary, albeit conflicting, data on this issue. Since the editorial in question, the full details of our own work5 have appeared.h In his editorial. Dr Landau once again3 described the biochemical capacity for isotopic exchange between acetoacetyl-CoA and labeled acetoacetate in muscle. He further supports his argument with “physiological” data from one of his own publications in hepatectomized dogs4 We do not deny the biochemical capacity for isotopic exchange.h We do, however, question the appropriateness of the hepatectomized dog, an animal with low ketogenic capacity, to reflect the circumstance in intact humans possessing a functioning liver. The precise issue that we investigateds,h was whether the mechanism of isotopic exchange contributed significantly to the tracer-measured appearance rate of acetoacetate across human muscle tissue. In part, these investigations were based on the detection of such exchange by the positional rearrangement or randomization of labeled acetoacetate carbon transversing muscle tissue. Such rearrangement is dictated by the biochemical scenario described by Dr Landau. We were unable to detect any randomization of labeled carbon. We concluded, therefore, that the exchange process did not contribute significantly to the results observed. In his editorial, Dr Landau minimizes our negative data by stating that “the isotopic exchanges cannot be equated with production or Surely, on the other hand, if we had shown such utilization.” exchange Dr Landau would have used it as support for his contention that isotopic exchange was the mechanism responsible for our thesis about the appearance of new ketone bodies across the human forearm. Apparently, the only data that will satisfy Dr Landau about the practical consequences of the theoretical potential for isotopic exchange are data that support his own position.
AN EXCHANGE
OF CONCLUSIONS
Further, to expand our understanding of this issue6 with experimental data rather than conjecture, we also infused normal adults with Intralipid plus heparin in order to raise the level of free fatty acid delivery to forearm muscle to a degree comparable to that seen in hyperglycemic diabetic adults with insulin insufficiency.h Via fatty acid oxidation, this maneuver should produce comparable intramuscular unlabeled acetoacetyl-CoA pool and, therefore, similar passive exchange dilution of labeled acetoacetate tracer across the muscle bed. Nonetheless, the degree of ketone body isotopic dilution across the forearm was vastly different between these groups. These data, while not unequivocal proof, support our own hypothesis but do not support Dr Landau’s contention. From these data, therefore, we concluded that simple isotopic exchange alone cannot fully account for the apparently increased rate of appearance of unlabeled ketone bodies measured across the forearm of diabetic individuals. In conclusion, while we appreciate continued discussion of the issues of isotopic exchange in tracer kinetics, we object to implications of the position espoused in the aforementioned Metabolism editorial, namely that we were unaware of the implications of such exchange on our results and that our data can be explained by exchange alone. In fact, our published experiment@ were designed to test the significance of the such exchange in human physiology and, in fact, provide data that support our position rather than Landau’s on this issue
Ramano Nosadini Department of Internal Medicine University of Padua, Italy Angelo Avogaro Department of Clinical Medicine University of Padua, Italy Dennis M. Bier Department of Pediatrics Washington University St Louis, MO
REFERENCES 1. Landau BR, Wahren J: Nonproductive exchanges: The use of isotopes gone astray. Metabolism 41:457, 1992 2. Nosadini R, Avogaro A, Sacca L. et al: Ketone body metabolism in normal and diabetic human skeletal muscle. Am J Physiol 249:E131-E136.1985 3. Landau BR: A potential pitfall in the use of isotopes to measure ketone body production. Metab Clin Exp. 35:94, 1986 4. Des Rosiers C, Montgomery JA, Garneau M. et al: Pseudoke-
togenesis in hepatectomized dogs. Am J Physiol 258:E519-E528, 1990 5. Avogaro A, Doria A, Valerio A, et al: Apparent ketone body “production” from the human forearm is not due to isotopic exchange via acetoacetyl-CoA thiolase. Diabetes 38:22A. 1989 (suppl2) 6. Avogaro A, Doria A, Gnudi L, et al: Forearm ketone body metabolism in normals, and insulin dependent diabetic patients. Am J Physiol263:E261-E267,1992
REPLY
necessary consequence of pseudoketogenesis is the transfer of label from [3,4-‘“CzJacetoacetate to form [1,2-“Czlacetoacetate, ie, randomization of labeled carbons, which they were unable to detect. Let me explain (1) why Avogaro et al’s interpretation of their data is erroneous, (2) why their data are explained by exchange alone, and (3) why their data confirm the existence of pseudoketogenesis with more occurring in muscles of diabetic subjects. First, please look at Fig 1 which shows the scheme used by Avogaro et al’ for their interpretation. Acetyl-CoA is not, as the scheme indicates. a necessary intermediate in the formation of acetoacetyl-CoA from fatty acids (FA). For Fig I to be correct, an arrow must be introduced from FA to acetoacetyl-CoA. That is so because the last four carbons of FA are directly converted to acetoacetyl-CoA6 (as depicted in references 5, 7, and 8 and shown in Fig 2). Thus, the biochemical scenario described by my col-
To the Editor: I welcome the opportunity to respond, with the concurrence of Drs Wahren and Brunengraber, to the comments directed toward me by Drs Nosadini, Avogaro, and Bier. now that full details of their work are available.’ Avogaro et alie investigated the metabolism of [3,4-r3Cs] acetoacetate in human forearm muscle. They observed net uptake of ketone bodies (KB) and dilution of i3C enrichment of KB across muscle, the extent of dilution varying with conditions. They interpret their data as evidence of KB production by muscle and more by diabetic muscle. They discount as an explanation pseudoof KB by isotopic ketogenesis, 4.s ie, dilution of the i3C enrichment exchange between acetoacetate and acetoacetyl-CoA. They claim a