Pyruvate dehydrogenase (PDH) decarboxylates pyruvate into acetyl-CoA and links glycolysis with the Krebs cycle. Because PDH is the only step where carbohydrate-derived substrate can enter the mitochondria and become completely oxidized, PDH activity can potentially determine if glycogen / glucose is oxidized completely, or whether pyruvate is converted to lactate.
Activity of PDH in the active form (PDHa) is overall determined by the degree of PDH-E1α phosphorylation, where PDH-E1α dephosphorylation activates PDH, while PDH-E1α phosphorylation inactivates PDH. The PDH-E1α phosphorylation state is determined by the overall content / activity of the regulatory proteins PDH kinase (PDK), of which there are 4 isoforms, and PDH phosphatase (PDP), of which there are 2 isoforms.
The overall aim of the PhD project was to elucidate 4 issues. 1: Role of muscle type in resting and exercise-induced PDH regulation in human skeletal muscle. 2: Effect of muscle glycogen on PDH regulation in human skeletal muscle at rest and during exercise. 3: The impact of physical inactivity on PDH regulation in human skeletal muscle at rest and during exercise. 4: Elucidating the importance of PGC-1α in PDH regulation in mouse skeletal muscle at rest and in response to fasting and during recovery from exercise. The studies indicate that the content of PDH-E1α in human muscle follows the metabolic profile of the muscle, rather than the myosin heavy chain fiber distribution of the muscle. The larger lactate accumulation in arm than leg muscles during exercise in humans may be the result of lower PDH-E1α content and not a muscle type dependent difference in PDH regulation. Both low muscle glycogen and increased plasma FFA are associated with upregulation of PDK4 protein and less exercise-induced increase in PDHa activity in human skeletal muscle. It may be noted that the increased PDK4 protein associated with elevated plasma FFA occurs already 2 hours after different dietary intake. A week of physical inactivity (bed rest), leading to whole body glucose intolerance, does not affect muscle PDH-E1α content, or the exercise-induced PDH regulation. Use of peroxisome proliferator activated receptor γ coactivator 1α (PGC-1α) knockout mice and muscle-specific PGC-1α overexpressing mice suggests that PGC-1α increases PDH-E1α content, but does not have clear effects on PDK4 protein content or regulation of PDHa activity in response to fasting and in recovery from exercise in mice.
Overall, there is a very close association between the phosphorylation degree of the 3 measured phosphorylation sites and the PDHa activity. This association is very strong at the onset of an exercise bout, but becomes weaker during the later part of an exhaustive high-intensity exercise bout, this indicates that additional regulatory mechanisms are involved in this phase of exercise. Mass spectrometry has identified a new phosphorylation site, along with 4 PDH-E1α-acetylation sites, which may have regulatory effects that can explain the current findings. This remains to be determined in future studies.