Lysyl oxidase (lox) gene deficiency affects osteoblastic phenotype

Lysyl oxidase (LOX) catalyzes cross-linking of elastin and collagen, which is essential for the structural integrity and function of bone tissue. The present study examined the role of Lox gene deficiency for the osteoblast phenotype in primary calvarial osteoblasts from E18.5 Lox knockout (Lox ( -/- )) and wild type (wt) (C57BL/6) mice. Next to Lox gene depletion, mRNA expression of Lox isoforms, LOXL1-4, was significantly downregulated in Lox ( -/- ) bone tissue. A significant decrease of DNA synthesis of Lox ( -/- ) osteoblasts compared to wt was found. Early stages of osteoblastic apoptosis studied by annexin-V binding as well as later stages of DNA fragmentation were not affected. However, mineral nodule formation and osteoblastic differentiation were markedly decreased, as revealed by significant downregulation of osteoblastic markers, type I collagen, bone sialoprotein, and Runx2/Cbfa1.

Multiple bone morphogenetic protein 1-related mammalian metalloproteinases process pro-lysyl oxidase at the correct physiological site and control lysyl oxidase activation in mouse embryo fibroblast cultures

Lysyl oxidase catalyzes the final enzymatic step required for collagen and elastin cross-linking in extracellular matrix biosynthesis. Pro-lysyl oxidase is processed by procollagen C-proteinase activity, which also removes the C-propeptides of procollagens I-III. The Bmp1 gene encodes two procollagen C-proteinases: bone morphogenetic protein 1 (BMP-1) and mammalian Tolloid (mTLD). Mammalian Tolloid-like (mTLL)-1 and -2 are two genetically distinct BMP-1-related proteinases, and mTLL-1 has been shown to have procollagen C-proteinase activity. The present study is the first to directly compare pro-lysyl oxidase processing by these four related proteinases. In vitro assays with purified recombinant enzymes show that all four proteinases productively cleave pro-lysyl oxidase at the correct physiological site but that BMP-1 is 3-, 15-, and 20-fold more efficient than mTLL-1, mTLL-2, and mTLD, respectively. To more directly assess the roles of BMP-1 and mTLL-1 in lysyl oxidase activation by connective tissue cells, fibroblasts cultured from Bmp1-null, Tll1-null, and Bmp1/Tll1 double null mouse embryos, thus lacking BMP-1/mTLD, mTLL-1, or all three enzymes, respectively, were assayed for lysyl oxidase enzyme activity and for accumulation of pro-lysyl oxidase and mature approximately 30-kDa lysyl oxidase. Wild type cells or cells singly null for Bmp1 or Tll1 all produced both pro-lysyl oxidase and processed lysyl oxidase at similar levels, indicating apparently normal levels of processing, consistent with enzyme activity data. In contrast, double null Bmp1/Tll1 cells produced predominantly unprocessed 50-kDa pro-lysyl oxidase and had lysyl oxidase enzyme activity diminished by 70% compared with wild type, Bmp1-null, and Tll1-null cells. Thus, the combination of BMP-1/mTLD and mTLL-1 is shown to be responsible for the majority of processing leading to activation of lysyl oxidase by murine embryonic fibroblasts, whereas in vitro studies identify pro-lysyl oxidase as the first known substrate for mTLL-2.

Oxidation of the His-52 --> Leu mutant of cytochrome c peroxidase by p-nitroperoxybenzoic acid: role of the distal histidine in hydroperoxide activation

Both cytochrome c peroxidase (CcP) and a mutant cytochrome c peroxidase in which the distal histidine has been replaced by leucine, CcP(H52L), are converted to hydroxy-ligated derivatives at alkaline pH. In CcP, the hydroxy-ligated derivative is subsequently converted to a bis-imidazole species prior to protein denaturation while the initial hydroxy-ligated CcP(H52L) is converted to a second, spectroscopically distinct hydroxy-ligated species prior to denaturation. The spectra of the alkaline forms of CcP and CcP(H52L) have been determined between 310 and 700 nm. The pH dependence of the rate of reaction between CcP(H52L) and hydrogen peroxide has been extended to pH 10. The hydroxy-ligated form of CcP(H52L) reacts with hydrogen peroxide 4 times more rapidly than the pentacoordinate, high-spin form of CcP(H52L) that exists at neutral pH. The rate of the reaction between p-nitroperoxybenzoic acid and CcP(H52L) has been measured between pH 4 and pH 8. Neutral p-nitroperoxybenzoic acid reacts with CcP(H52L) 10(5) times more slowly than with CcP while the negatively charged p-nitroperoxybenzoate reacts with CcP(H52L) 10(3) times more slowly than with CcP. These data indicate that the role of the distal histidine during the initial formation of the peroxy anion/heme iron complex is not simply base catalysis.

Oxidation of cytochrome c peroxidase to compound I by peroxyacids: evidence for rate-limiting diffusion through the protein matrix

The rate of the reaction between p-nitroperoxybenzoic acid and cytochrome c peroxidase (CcP) has been investigated as a function of pH and ionic strength. The pH dependence of the reaction between CcP and peracetic acid has also been determined. The rate of the reactions are influenced by two heme-linked ionizations in the protein. The enzyme is active when His-52 (pK(a) 3.8 +/- 0.1) is unprotonated and an unknown group with a pK(a) of 9.8 +/- 0.1 is protonated. The bimolecular rate constant for the reaction between peracetic acid and CcP and between p-nitroperoxybenzoic acid and CcP are (1.8 +/- 0.1) x 10(7) and (1.6 +/- 0.2) x 10(7) M(-)(1) s(-)(1), respectively. These rates are about 60% slower than the reaction between hydrogen peroxide and CcP. A critical comparison of the pH dependence of the reactions of hydrogen peroxide, peracetic acid, and p-nitroperoxybenzoic acid with CcP provides evidence that both the neutral and anionic forms of the two peroxyacids react directly with the enzyme. The peracetate and p-nitroperoxybenzoate anions react with CcP with rates of (1.5 +/- 0.1) x 10(6) and (1.6 +/- 0.2) x 10(6) M(-)(1) s(-)(1), respectively, about 10 times slower than the neutral peroxyacids. These data indicate that CcP discriminates between the neutral peroxyacids and their negatively charged ions. However, the apparent bimolecular rate constant for reaction between p-nitroperoxybenzoate and CcP is independent of ionic strength in the range of 0.01-1.0 M, suggesting that electrostatic repulsion between the anion and CcP is not the cause of the lower reactivity for the peroxybenzoate anion. The data are consistent with the hypothesis that the rate-limiting step for the oxidation of CcP to compound I by both neutral peroxyacid and the negatively charged peroxide ion is diffusion of the reactants through the protein matrix, from the surface of the protein to the distal heme pocket.