Protein lysine 6-oxidase (lysyl oxidase) cofactor: methoxatin (PQQ) or pyridoxal?

Lysyl oxidase: mechanism, regulation and relationship to liver fibrosis

Lysyl oxidase oxidizes peptidyl lysine in collagen and elastin substrates to residues of alpha-aminoadipic-delta-semialdehyde. The peptidyl aldehydes can then undergo spontaneous condensations with unreacted epsilon-amino groups and with neighboring aldehyde functions, thus forming the covalent crosslinkages which convert elastin and collagen into insoluble fibers. The unique role of lysyl oxidase in the post-translational modification of these proteins qualifies this enzymatic reaction as a potentially pivotal site of biological and/or chemotherapeutic control of collagen fiber deposition. Recent advances in the study of the catalytic mechanism, in the development of active site inhibitors, and in the biosynthesis and regulation of this unusual catalyst are reviewed as are studies on the response of lysyl oxidase in fibrotic liver.

The pyrroloquinoline quinone (PQQ) coenzymes: a case of mistaken identity

Recent evidence has failed to support the claim for a pyrroloquinoline quinone (PQQ) cofactor in mammalian enzymes previously reported to have PQQ. The validity of the original analysis now has been questioned, and a second cofactor, topa quinone, has been identified in at least one enzyme.

Properties and function of lysyl oxidase

Lysyl oxidase catalyzes the oxidation of peptidyl lysine to alpha-aminoadipic-delta-semialdehyde, the precursor to the covalent crosslinkages that stabilize fibers of elastin and collagen. This enzyme contains both copper and a carbonyl cofactor consistent with an o-quinone. The proposed mechanism of action is derived from available kinetic and chemical data and also can account for mechanism-based inhibition of the enzyme by specific monoamines and diamines. Recent evidence for biosynthetic precursors and for the regulation of lysyl oxidase in fibrotic and malignant diseases is discussed.

Quinoproteins: enzymes containing the quinonoid cofactor pyrroloquinoline quinone, topaquinone or tryptophan-tryptophan quinone

The presently best known and largest group of quinoproteins consists of enzymes using the cofactor 2,7,9-tricarboxy-1H-pyrrolo[2,3-f]quinoline- 4,5-dione (PQQ), a compound having a pyrrole ring fused to a quinoline ring with an o-quinone group in it. Representatives of this group are found among the bacterial, NAD(P)-independent, periplasmic dehydrogenases. Despite their high midpoint redox potential, the overall behaviour of quinoprotein dehydrogenases is similar to that of their counterparts, those using a flavin cofactor or a nicotinamide coenzyme. Apart from an exceptional Gram-positive one, the sole organisms where the presence of PQQ has really been established are Gram-negative bacteria. Evidence for the occurrence of covalently bound PQQ is lacking since it has now been shown that several enzymes previously considered to contain this prosthetic group do not in fact do so. Another group of quinoproteins, consisting of amine oxidoreductases, has a protein chain containing one of the following quinonoid aromatic amino acids: 6-hydroxy-phenylalanine-3,4-dione (TPQ) or 4-(2'-tryptophyl)-tryptophan-6,7-dione (TTQ). There is no doubt that these o-quinones play a role as cofactor, in the case of TPQ in prokaryotic as well as eukaryotic amine oxidases. It appears, therefore, that a novel class of amino-acid-derived cofactors is emerging, ranging from the free radical form of tyrosine and tryptophan to those containing a dicarbonyl group (like the already known pyryvoyl group and the o-quinones here described.

Skin wound healing in riboflavin deficiency

Healing of excision and incision wounds was evaluated in riboflavin-deficient rats. The period taken for the epithelialization of excision wounds was 4 to 5 days longer in riboflavin-deficient animals compared to ad libitum-fed or food-restricted weight-matched control groups. Riboflavin deficiency as well as food restriction slowed the rate of wound contraction, the effect of riboflavin deficiency being of greater magnitude. The tensile strength of incision wounds in riboflavin deficiency was reduced to 42% of the ad libitum-fed control and 63% of the weight-matched control values. There was a decrease of 25% in total collagen content of incision wounds, in riboflavin deficiency and its maturity was drastically affected as indicated by a twofold increase in salt solubility (1 M NaCl) and a four-fold increase in the alpha/beta subunit ratio of salt-soluble collagen. Food restriction had similar effects but of lower magnitude. The data suggest that alteration in collagen content and maturity may be responsible for the lower tensile strength of incision wounds in riboflavin-deficient rats. This suggestion was supported by the results of a rehabilitation experiment.