A low molecular weight bovine kidney acid phosphatase, electrophoretically homogeneous and with a relative molecular mass of 17.8 kDa, was used in this work. Among the various substrates tested, FMN was found to be the most effective, at pH 7.0. Distinct activation energy values were obtained for p‐nitrophenyl phosphate‐ (45.44 kJ mol‐1) and flavin mononucleotide‐ (28.60 kJ mol‐1) hydrolysis reactions. The FMN hydrolysis was strongly inhibited by Cu2+ and pCMB, but activated by guanosine. Pyridoxal‐phosphate and vanadate were competitive inhibitors for the FMN‐dependent reaction.

Recebido em 17/8/04; aceito em 5/11/04; publicado na web em 13/4/05 RIBOFLAVIN: A MULTIFUNCTIONAL VITAMIN. Riboflavin, a component of the B 2 vitaminic complex, plays important roles in biochemistry, especially in redox reactions, due to the ability to participate in both one-and two-electron transfers as well as acting as a photosensitizer. Accordingly, low intakes of this vitamin have been associated with different diseases, including cancer and cardiovascular diseases. Riboflavin is thought to contribute to oxidative stress through its capacity to produce superoxide but, interestingly, it can also promote the reduction of hydroperoxides. This peculiar and multifunctional behavior allows riboflavin to take part in various biochemical pathways as a nucleophile and an electrophile, turning it into a versatile and important biological compound.Keywords: riboflavin; electron transfer; oxidative stress. INTRODUÇÃOA riboflavina foi isolada (apesar de não purificada) pela primeira vez em 1879 pelo químico inglês A. Wynter Blyth. Em seus estudos relacionados à composição do leite de vaca ele relatara a identificação de um pigmento amarelo brilhante que chamou de "lactocromo" e que hoje nós conhecemos como sendo a riboflavina. Depois desse anúncio, quase cinqüenta anos se passaram até que algum avanço significativo ocorresse com o recém descoberto pigmento amarelo-laranja. Então, ao final dos anos 20 e início da dé-cada de 30 do século XX, uma grande evolução científica aconteceu e pigmentos amarelos semelhantes com um brilho esverdeado fluorescente foram isolados de uma ampla variedade de fontes 1 . O interesse por eles tornou-se ainda maior quando o tal pigmento amarelo foi reconhecido como sendo um constituinte da vitamina do complexo B, sendo a purificação da própria vitamina amplamente facilitada uma vez que se percebeu que o potencial vitamínico estava intimamente correlacionado à fluorescência da riboflavina. Alguns dos principais químicos da época como Richard Kuhn, em Heidelberg, e Paul Karrer, em Zurich, entraram em uma corrida acirrada para determinar a estrutura, bem como para prová-la através de síntese química, sendo que ambos obtiveram sucesso neste intento quase que concomitantemente. Vários nomes foram propostos para o então caracterizado composto, tais como lactoflavina e ovoflavina a fim de estabelecer correlação com a fonte através da qual o composto havia sido isolado. Mas o prefixo ribo foi consagrado no nome pelo qual conhecemos o composto atualmente, por causa da cadeia ribitil e da cor amarela característica proveniente do sistema π conjugado nos anéis isoaloxazina [1][2][3] . RIBOFLAVINA E SUAS FUNÇÕES BIOLÓGICASA riboflavina, 7,8-dimetil-10-ribitil-isoaloxazina, é uma vitamina hidrossolúvel pertencente ao complexo vitamínico B 2 , apresenta coloração amarela e é fluorescente. Além do leite que, como mencionado, foi uma das primeiras fontes de obtenção, a riboflavina é encontrada também em carne, peixe e, principalmente, em vegetais de cor verde-escura 4 . A riboflavina proveniente da dieta en...

Wheat germ DNA polymerase A, a gamma-like enzyme, recognized efficiently natural and synthetic RNA templates, resembling a retroviral reverse transcriptase (P. Laquel et al., Biochim Biophys Acta 1048 (1990): 139-148). Ammonium-21-tungsto-9-antimoniate (HPA-23), an antiviral drug, inhibited the DNA polymerase A activities, independently of the template primers used, i.e. activated DNA or polyriboadenylic acid oligodeoxythymidylate (poly(rA)-oligo(dT)). The inhibition observed in the poly(rA)-oligo(dT)-directed DNA polymerase A activity occurred in the presence of either Mg2+ or Mn2+ as divalent cation, and also with the 2'-fluoro analogue of poly(rA) as template. HPA-23 was a non-competitive inhibitor with respect to TTP, activated DNA, poly(rA)-oligo(dT), and poly(dAfl)-oligo(dT). A preincubation study showed a reversible HPA-23 binding to DNA polymerase A, in the presence of poly(rA)-oligo(dT) as the template primer.

Some intercalating and nonintercalating drugs have been tested as inhibitors on the DNA synthesis reaction catalyzed by avian myeloblastosis virus (AMV) reverse transcriptase, in the presence of polyriboadenylic acid (poly(rA)) and poly(2'-fluoro-2'-deoxyadenylic acid) (poly(dAfl)) as templates. In both cases, the inhibition was higher with the intercalating drug ethidium bromide than with the nonintercalating analog tetramethyl ethidium bromide. Ethidium bromide inhibited more efficiently the poly(rA)- than the poly(dAfl)-directed reverse transcriptase reaction; in the latter case, the inhibition was non-competitive in relation to TTP. On the other hand, the reaction catalyzed in the presence of the 2'-fluorinated polynucleotide as template was inhibited to a higher extent by other nonintercalating drugs, berenil, netropsin, and distamycin. The inhibitions of both reactions by dideoxy TTP, novobiocin and HPA-23 are also discussed.

No abstract

Several homologous polynucleotides have been tested as inhibitors on the reactions catalyzed by avian myeloblastosis virus (AMV) reverse transcriptase, in the presence of polyribonucleotides and 2'-fluorinated polynucleotides as templates. Polynucleotides differentially inhibited the reactions catalyzed by reverse transcriptase in the presence of these synthetic templates. Polyriboadenylic acid (poly(rA), poly(2'-O-methyladenylic acid) (poly(Am)), poly(2'-fluoro-2'-deoxyadenylic acid) (poly(dAfl), polyinosinic acid (poly(rI)) and polyuridylic acid poly(rU)) inhibited the polyribonucleotide-, but not the 2'-fluorinated polynucleotide-directed reverse transcriptase activity.