Degradation of Hyaluronic Acid by Photosensitized Riboflavin In Vitro. Modulation of the Effect by Transition Metals, Radical Quenchers, and Metal Chelators

The effect of photoexcited riboflavin (RF) on the viscosity of hyaluronic acid (HA) solutions has been investigated. UV irradiation of RF causes under aerobic conditions fragmentation of HA and a decrease in the viscosity of its solutions. A decrease of HA viscosity occurs in PO₄-buffered solutions and is accelerated by high pH, Fe²⁺ (but much less so by Fe³⁺), certain metal chelators, and horseradish peroxidase (HRP); it is partially inhibited by catalase and less so by superoxide dismutase (SOD). The reactivity of the system was completely blocked by Tris, ethanol, aspirin, d-manitol, dimethylthiourea (DMTU), dimethylsulfoxide (DMSO), and sodium azide. These results indicate that the most likely chemical species involved in the reaction is the hydroxyl radical. Singlet oxygen (¹0₂) generation is suggested by the ability of NaN₃ and DMSO to completely inhibit the reactivity of the system. These two agents, however, may also interact with OH^√ radical, as well and suppress the reactivity of the system. H₂O₂ and seem also to be produced in significant amounts, because catalase and SOD partially block the reactivity of the system. The effect of HRP may be due to hydrogen subtraction from HA and H₂O₂ reduction to water. Photoexcitation of RF may potentially occur in vitro and in vivo in the organs and tissues that are permeable to light, such as the eye or skin, and damage HA and other cell-matrix components causing inflammation and accelerating aging.  © 1997 Elsevier Science Inc.     

Characterization of Zea mays endosperm C-24 sterol methyltransferase: one of two types of sterol methyltransferase in higher plants

We report the characterization of a higher-plant C-24 sterol methyltransferase by yeast complementation. A Zea mays endosperm expressed sequence tag (EST) was identified which, upon complete sequencing, showed 46% identity to the yeast C-24 methyltransferase gene (ERG6) and 75% and 37% amino acid identity to recently isolated higher-plant sterol methyltransferases from soybean and Arabidopsis, respectively. When placed under GAL4 regulation, the Z. mays cDNA functionally complemented the erg6 mutation, restoring ergosterol production and conferring resistance to cycloheximide. Complementation was both plasmid-dependent and galactose-inducible. The Z. mays cDNA clone contains an open reading frame encoding a 40 kDa protein containing motifs common to a large number of S-adenosyl-L-methionine methyltransferases (SMTs). Sequence comparisons and functional studies of the maize, soybean and Arabidopsis cDNAs indicates two types of C-24 SMTs exist in higher plants.     

Inhibitors of histone methylation

Two classes of inhibitors of histone methyltransferase I from calf thymus are reported. High concentrations ($\text{≧ 10}\text{m}\text{M}$) of various alkyl or aralkyl amines and polyamines were inhibitory to the enzyme. Spermine and spermidine were among the most potent compounds in this group. The best monoamine inhibitor was 2-phenylethylamine, which gave 47% inhibition at 10 mM.The substituted phenanthridinium compound ethidium bromide was also an inhibitor of the enzyme. A number of analogs of ethidium bromide were tested, and the most potent compound (17) gave 50% inhibition at 0.125 mM. $\text{S-}\text{Adenosyl}\text{-}\text{l}\text{-}\text{ethionine}$ (SAM) showed competitive inhibition of the enzyme as determined from a Lineweaver-Burke plot, while ethidium bromide was noncompetitive.