FIBER-TYPE-RELATED DIFFERENCES IN THE ENZYMES OF A PROPOSED SUBSTRATECYCLE

Citation
Ra. Howlett et Wt. Willis, FIBER-TYPE-RELATED DIFFERENCES IN THE ENZYMES OF A PROPOSED SUBSTRATECYCLE, Biochimica et biophysica acta. Bioenergetics, 1363(3), 1998, pp. 224-230
Citations number
28
Categorie Soggetti
Biology,Biophysics
ISSN journal
00052728
Volume
1363
Issue
3
Year of publication
1998
Pages
224 - 230
Database
ISI
SICI code
0005-2728(1998)1363:3<224:FDITEO>2.0.ZU;2-8
Abstract
A substrate cycle between citric acid cycle (CAC) intermediates isocit rate and 2-oxoglutarate, involving NAD(+)- and NADP(+)-linked isocitra te dehydrogenase (NAD-IDH and NADP-IDH, respectively) and mitochondria l transhydrogenase (H+-Thase), has recently been proposed. This cycle has been hypothesized to enhance mito-chondrial respiratory control by increasing the sensitivity of NAD-IDH to its modulators and allowing for enhanced increases in flux through this step of the CAC during per iods of increased ATP demand. The activities of the enzymes comprising the substrate cycle: NAD-IDH, forward and reverse NADP-IDH. and forwa rd and reverse H+-Thase, along with the activity of a marker of mitoch ondrial content, citrate synthase (CS) were measured in mitochondria i solated from rabbit Type I and Type IIb muscles and in whole muscle ho mogenates, representing the various fiber types, from rats. In isolate d rabbit muscle mitochondria, NAD-IDH had significantly higher (1.6 X) activity in white muscle while forward NADP-IDH, forward and reverse H+-Thase. and CS all had significantly higher (1.3-1.6 X) activities i n red muscle, There was no difference in reverse NADP-IDH between fibe r types. Similarly, in rat whole muscle enzyme activities normalized t o CS, NAD-IDH had significantly higher activity in fast-twitch glycoly tic (FG) fibers, while forward NADP-IDH and forward H+-Thase had signi ficantly higher activities in slow-twitch oxidative (SO) fibers. These results suggest that differences in the activities of the substrate c ycle enzymes between skeletal muscle fiber types could contribute to d ifferences in respiratory control due to differential cycling rates an d/or loci of control. (C) 1998 Elsevier Science B.V.