Oxidative folding: recent developments

The Level of ALR is Regulated by the Quantity of Mitochondrial DNA

Augmenter of liver regeneration (ALR) contributes to mitochondrial biogenesis, maintenance and to the physiological operation of mitochondria. The depletion of ALR has been widely studied and had serious consequences on the mitochondrial functions. However the inverse direction, the effect of the depletion of mitochondrial electron transfer chain and mtDNA on ALR expression has not been investigated yet. Thus mtDNA depleted, ρ(0) cell line was prepared to investigate the role of mitochondrial electron transfer chain and mtDNA on ALR expression. The depletion of mtDNA has not caused any difference at mRNA level, but at protein level the expression of ALR has been markedly increased. The regulatory role of ATP and ROS levels could be ruled out because the treatment of the parental cell line with different respiratory inhibitors and uncoupling agent could not provoke any changes in the protein level of ALR. The effect of mtDNA depletion on the protein level of ALR has been proved not to be liver specific, since the phenomenon could be observed in the case of two other, non-hepatic cell lines. It seems the level of mtDNA and/or its products may have regulatory role on the protein level of ALR. The up-regulation of ALR can be a part of the adaptive response in ρ(0) cells that preserves the structural integrity and the transmembrane potential despite the absence of protein components encoded by the mtDNA.

ALR, the multifunctional protein

ALR is a mystic protein. It has a so called “long” 22 kDa and a “short” 15 kDa forms. It has been described after partial hepatectomy and it has just been considered as a key protein of liver regeneration. At the beginning of the 21st century it has been revealed that the “long” form is localized in the mitochondrial intermembrane space and it is an element of the mitochondrial protein import and disulphide relay system. Several proteins of the substrates of the mitochondrial disulphide relay system are necessary for the proper function of the mitochondria, thus any mutation of the ALR gene leads to mitochondrial diseases. The “short” form of ALR functions as a secreted extracellular growth factor and it promotes the protection, regeneration and proliferation of hepatocytes. The results gained on the recently generated conditional ALR mutant mice suggest that ALR can play an important role in the pathogenesis of alcoholic and non-alcoholic steatosis. Since the serum level of ALR is modified in several liver diseases it can be a promising marker molecule in laboratory diagnostics. Orv. Hetil., 2015, 156(13), 503–509.

The role of ascorbate in protein folding

Ascorbate was linked to protein folding a long time ago. At the first level of this connection, it had been shown that ascorbate functions as an essential cofactor in the hydroxylation enzymes involved in collagen synthesis. Although the hydroxylation reactions catalyzed by the members of the prolyl 4-hydroxylase family are considered to be ascorbate dependent, the hydroxylation of proline alone does not need ascorbate. Prolyl 4-hydroxylases participate in two catalytic reactions: one in which proline residues are hydroxylated, while 2-oxoglutarate is decarboxylated and molecular oxygen is consumed. This reaction is ascorbate independent. However, in another reaction, prolyl 4-hydroxylases catalyze the decarboxylation of 2-oxoglutarate uncoupled from proline hydroxylation but still needing molecular oxygen. At this time, ferrous iron is oxidized and the protein is rendered catalytically inactive until reduced by ascorbate. At the second level of the connection, the oxidation and the oxidized form of ascorbate, dehydroascorbate, is involved in the formation of disulfide bonds of secretory proteins. The significance of the dehydroascorbate reductase activity of protein disulfide isomerase was debated because protein disulfide isomerase as a dehydroascorbate reductase was found to be too slow to be the major route for the reduction of dehydroascorbate (and formation of disulfides) in the endoplasmic reticulum lumen. However, very recently, low tissue ascorbate levels and a noncanonical scurvy were observed in endoplasmic reticulum thiol oxidase- and peroxiredoxin 4-compromised mice. This novel observation implies that ascorbate may be involved in oxidative protein folding and creates a link between the disulfide bond formation (oxidative protein folding) and hydroxylation.