Influence of subunit transcript and protein levels on formation of a mitochondrial multienzyme complex

Abstract

Constitutive expression of nuclear genes encoding mitochondrial proteins raises the question of whether these proteins are present in similar amounts in mitochondria of different tissues. We report that amounts of a single multienzyme complex can vary on a per mitochondrion basis depending on the number of mitochondria per cell. Human branched-chain a-keto acid dehydrogenase (BCKD) expression is used as a paradigm in these studies. Expression is compared and contrasted in HepG2 and DG75 cells in which mitochondrial content is twofold higher in the hepatocarcinoma line than in the lymphoblastoid line. Per cell, BCKD activity is equal in the two cell types, but BCKD protein concentration per mitochondrion is twofold higher in DG75 cells. Steady-state mRNA levels do not appear to be directly related to amounts of protein in the two cell lines. To test whether one subunit is limiting in formation of complex, overexpression of each BCKD subunit was elicited by plasmid transfection of the DG75 cells. Only overexpression of the p-subunit of the decarboxylase component induced more BCKD activity without apparent increase in mRNA for the other endogenously expressed subunits. This implies that free BCKD subunits exist in a cell and can be recruited into an active complex when the limiting subunit becomes available.

Role of Branched‐Chain Aminotransferase Isoenzymes and Gabapentin in Neurotransmitter Metabolism

“…Because the active form of El is a tetramer of 2cr and 2~subunits, the data suggest that net complex activity in astrocyte cultures may be limited by availability of the El~subunit. McConnell et al (1996) have also reported that levels of thes ubunit can limit BCKD complex activity. Other known regulatory factors include phosphorylation state regulated by the BCKD kinase (Harris et al, 1986) and product inhibition (acyl-CoA intermediates and NADH).…”

Human mutations affecting branched chain a-ketoacid dehydrogenase

“…Available data suggest that the import of E1β can alter the amount of E1 subunits within a mitochondrion (46,72,73). Likewise, insulin appears to influence the production of E1α which could help to determine the amount of E1 present at any one time and glucocorticoids have been shown to alter E2 and E1β concentration (74).…”