The role of phosphoenolpyruvate carboxykinase in muscle alanine synthesis.
3-Mercaptopicolinic acid (3-MPA) is reportedly a specific inhibitor of phosphoenolpyruvate (PEP) carboxykinase and has hitherto been used accordingly to elucidate the metabolic role of PEP carboxykinase in vitro and in vivo. We show that 3-MPA has multiple effects on intermediary metabolism in hemidiaphragms from 40 h-starved rats. It decreases the release of lactate + pyruvate and alanine in hemidiaphragms provided with no added substrate or with valine, leucine or isoleucine. Moreover, irrespective of the substrate provided (none, valine, leucine, isoleucine, glucose, acetate, oleate), 3-MPA decreases the [lactate]/[pyruvate] ratio. 3-MPA is without effect on 14CO2 production from [U-14C]valine, [1-14C]valine, [1-14C]leucine, [U-14C]isoleucine or [1-14C]oleate, but stimulates 14CO2 production from [U-14C]glucose and [1-14C]pyruvate and inhibits 14CO2 production from [1-14C]acetate. Glycolytic flux (measured as 3H2O formation from [5-3H]glucose) is stimulated by 3-MPA. It is concluded that 3-MPA has site(s) of actions other than PEP carboxykinase and that the putative role of PEP carboxykinase in alanine synthesis de novo in skeletal muscle from tricarboxylic acid-cycle intermediates and related amino acids requires reappraisal.
D000409:Alanine / Q000096:biosynthesis*; D000818:Animals; D005971:Glutamates / Q000378:metabolism; D018698:Glutamic Acid; D005973:Glutamine / Q000378:metabolism; D066298:In Vitro Techniques; D007773:Lactates / Q000378:metabolism; D019344:Lactic Acid; D008297:Male; D009132:Muscles / Q000201:enzymology*; D010729:Phosphoenolpyruvate Carboxykinase (GTP) / Q000037:antagonists & inhibitors / Q000378:metabolism*; D010848:Picolinic Acids / Q000494:pharmacology; D011773:Pyruvates / Q000378:metabolism; D019289:Pyruvic Acid; D051381:Rats; D011919:Rats, Inbred Strains; D013217:Starvation / Q000201:enzymology; D013379:Substrate Specificity
D005971:Glutamates; D007773:Lactates; D010848:Picolinic Acids; D011773:Pyruvates; D005973:Glutamine; D019344:Lactic Acid; D018698:Glutamic Acid; C008363:3-mercaptopicolinic acid; D019289:Pyruvic Acid; D010729:Phosphoenolpyruvate Carboxykinase (GTP); D000409:Alanine