Oxygen uptake in the vitamin B-sensitized photo-oxidation of tyrosine and tryptophan in the presence of uracil: kinetics and mechanism.

Considering the significance of visible light-promoted reactions in complex biological media, the photo-oxidation of the amino acids (AAs) tyrosine (tyr) and tryptophan (trp) was studied in the presence of the naturally occurring oxidative scavenger uracil (ur). The involved photoprocesses, studied at pH 7 and 9, are driven through the reactive oxygen species (ROS) singlet molecular oxygen (O2(1Deltag)), superoxide radical anion (O2*-) and hydrogen peroxide (H2O2). The effect on the effectiveness of the overall photo-oxidation process due to the presence of an added electron-donating substrate such as ur is not straightforwardly predictable. The addition of the pyrimidine compound, a much lesser photo-oxidizable substrate than the AAs themselves, produced different results: (1) antioxidative for tyr at pH 9, decreasing the overall rate of oxygen uptake; (2) synergistic for tyr at pH 7, increasing the oxidation rate more than the corresponding addition value of the respective individual rates and (3) no effect for trp at both pH values. The final result depends on the respective abilities of the substrates as quenchers of both the long-lived riboflavin triplet excited state and the generated ROS and the pH of the medium. An interpretation for the different cases is attempted through a kinetic and mechanistic analysis.

Kinetics and mechanism of the sensitized photodegradation of uracil--modeling the fate of related herbicides in aqueous environments.

The dye-sensitized photodegradation of uracil (UR), the parent compound of several profusely employed herbicides, has been studied as a model of their environmental fate. In order to mimic conditions frequently found in nature, aqueous solutions of UR have been irradiated with visible light in the presence of the natural sensitizer riboflavin (Rf). The results indicate that UR is photostable in acid media, but is quickly degraded in pH 7 or pH 9 solutions, where singlet molecular oxygen [O2(1Delta(g))] and, to a lesser extent, superoxide radical anion (O2*-)-both species photogenerated from triplet excited Rf, 3Rf*-participate in the photodegradation. At pH 7, UR is slowly degraded through an O2*- -mediated mechanism, whereas Rf disappears through its reaction with O2(1Delta(g)) and, in the form of 3Rf*, with UR. On the contrary, at pH 9 Rf is photoprotected through two processes: its regeneration from the formed Rf radical species-a back electron transfer that also produces O2*- -and the elimination from the medium of O2(1Delta(g)) by its reaction with UR. The overall result of the preservation of ground state Rf is the continuity of the photosensitized process and, hence, of the UR degradation. Media with higher pH values could not be employed due to the fast photodegradation of Rf. With rose bengal (RB) as photosensitizer, the rate constants found for the overall interaction between UR and the photogenerated O2(1Delta(g)) were in the range 5 x 10(5) M(-1) s(-1) (at pH 7) to 1.3 x 10(8) M(-1) s(-1) (in 1 M NaOH aqueous solution, mainly physical quenching). The maximum O2(1Delta(g)0-mediated photooxidation efficiencies with RB were reached at pH 11, where only the O2(1Delta(g)0-reactive quenching with UR was observed.

Modelling the environmental degradation of water contaminants. Kinetics and mechanism of the riboflavin-sensitised-photooxidation of phenolic compounds.

The aerobic visible-light-photosensitised irradiation of methanolic solutions of either of the phenolic-type contaminants model compounds (ArOH) p-phenylphenol (PP), p-nitrophenol (NP) and phenol (Ph), and for two additional phenolic derivatives, namely p-chlorophenol (ClP) and p-methoxyphenol (MeOP), used in some experiments, was carried out. Employing the natural pigment riboflavin (Rf) as a sensitiser, the degradation of both the ArOH and the very sensitiser was observed. A complex mechanism, common for all the ArOH studied, operates. It involves superoxide radical anion (O2-*) and singlet molecular oxygen (O2(1delta(g)) reactions. Maintaining Rf in sensitising concentrations levels (approximately 0.02 mM), the mechanism is highly dependent on the concentration of the ArOH. Kinetic experiments of oxygen and substrate consumption, static fluorescence, laser flash photolysis and time-resolved phosophorescence detection of O2(1delta(g)) demonstrate that at ArOH concentrations in the order of 10 mM, no chemical transformation occurs due to the complete quenching of Rf singlet excited state. When ArOH is present in concentrations in the order of mM or lower, O2-* is generated from the corresponding Rf radical anion, which is produced by electron transfer reaction from the ArOH to triplet excited Rf. The determined reaction rate constants for this step show a fairly good correlation with the electron-donor capabilities for Ph, PP, NP, ClP and MeOP. In this context, the main oxidative species is O2-*, since O2(1delta(g)) is quenched in an exclusive physical fashion by the ArOH. The production of O2-* regenerates Rf impeding the total degradation of the sensitiser. This kinetic scheme could partially model the fate of ArOH in aquatic media containing natural photosensitisers, under environmental conditions.

On the generation and quenching of reactive-oxygen-species by aqueous vitamin B2 and serotonin under visible-light irradiation.

It is well known that endogenous daylight-absorbing compounds produce the sensitized photodegradation of biologically relevant substrates. In this context the photostability of a mixture of the indole neurotransmitter serotonin (Sero) and vitamin B2 (riboflavin, Rf) upon visible-light irradiation and the possible role of Sero and related compounds as generators or deactivators of reactive oxygen species (ROS) was investigated through a kinetic and mechanistic study. The work was done at pH 7 and under experimental conditions in which only the vitamin absorbs photoirradiation. Tryptamine (Trpa) and 5-hydroxyindole (OHIn) were included in the study as model compounds for the neurotransmitter. The visible light irradiation of aqueous Rf in the individual presence of Sero, Trpa and 5-OHIn, under aerobic conditions, induce degradative processes on the indole derivatives (In-der). At least two different mechanisms operate. Our analysis shows that the main reaction pathway is an electron-transfer-mediated quenching of triplet excited Rf ((3)Rf(*)) by the In-der. It produces the species Rf(-)/RfH() and the In-der radical cation that could react to form phenoxy and α-amino radicals. In a further reaction step the species O(2)(-) and OH() could be produced. In parallel, energy transfer from (3)Rf(*) to dissolved oxygen would generate O(2)((1)Δ(g)). Within the frame of the proposed mechanism, results suggest that Rf-sensitized degradation of Sero occurs via the mentioned ROS and non-oxygenated radical-mediated processes. The indole compound quenches O(2)((1)Δ(g)) in a dominant physical fashion. This fact constitutes a desirable property in antioxidants, provided that the quenching process practically does not eliminate the scavenger. Sero exerts a photoprotective effect towards tryptophan through the combined quenching of O(2)((1)Δ(g)) and (3)Rf(*), the latter excited species responsible for the generation of ROS. The amino acid can be taken as a target model of oxidizable biological substrates, particularly proteins.

Novel interpretación de los mecanismos que regulan la actividad de las células lactotropas / New interpretation of the mechanisms that regulate the activity of lactotropic cells

La correlación de microscopía, morfometría y RIA de prolactina en la hipófisis de la rata fue utilizada en el estudio bajo condiciones extremas de actividad secretoria. Durante la lactancia se observó una hipertrofia e hiperplasia de las células lactotropas, situación que es revertida con la interrupción de la lactancia. La generación masiva de células lactotropas en ratas ablactadas indican la presencia de células excedentes que son eliminadas cuando los requerimientos establecidos por una situación de excepción es suprimida. Un nuevo modelo de regulación de la actividad secretoria de las células de prolactina es propuesto (AU)

Novel interpretación de los mecanismos que regulan la actividad de las células lactotropas / New interpretation of the mechanisms that regulate the activity of lactotropic cells

La correlación de microscopía, morfometría y RIA de prolactina en la hipófisis de la rata fue utilizada en el estudio bajo condiciones extremas de actividad secretoria. Durante la lactancia se observó una hipertrofia e hiperplasia de las células lactotropas, situación que es revertida con la interrupción de la lactancia. La generación masiva de células lactotropas en ratas ablactadas indican la presencia de células excedentes que son eliminadas cuando los requerimientos establecidos por una situación de excepción es suprimida. Un nuevo modelo de regulación de la actividad secretoria de las células de prolactina es propuesto