Lead acetate Leucine

The effects of lead acetate closely resemble those reported with L-leucine.213 In each case, the compound itself does not appear to inhibit in vitro -tryptophan binding to hepatic nuclei. However, when each is added together with unlabeled L-tryptophan, it abrogates the inhibiting effect that the unlabeled L-tryptophan alone had on binding. This type of reaction pattern seems to be compatible with a response of an allosteric nature. Currently, no explanation for the effect of either compound is available. 

The presence of leucin and tyrosin in the urine may be detected as follows the freshly collected urine is treated with basic lead acetat filtered, the filtrate treated with HaS, filtered from the precipitated lead sulfid, and the filtrate evaporated over the water-bath leucin and tyrosin crystallize they may be separated by extraction of the residue with hot alcohol, which dissolves the leucin and leaves the tyrosin. The leucin left by evaporation of the alcoholic solution may be recognized by its crystalline form and by the following characters (1) a small portion is moistened on platinum foil with HNOa, which is then cautiously evaporated a colorless residue remains, which, when warmed with caustic soda solution, turns yeUow or brown, and by further concentration is converted into oily drops, which do not adhere to the platinum (Scherer s test) (2) a portion of the residue is heated in a dry test-tube it melts into oily drops, and the odor ofainylamin (odor of ammonia combined with that of fusel oil) is observed (3) if a boiling mixture of leucin and solution of neutral lead acetate be carefully neutralized with ammonia, brilliant crystals of a compound of leucin and lead oxid separate (4) leucin carefully heated in a glass tube, open at both ends, to 170° (338° F.), sublimes without fusing, and condenses in flocculent shreds. If heated beyond 180° (356° F.), the decomposition mentioned in (2) occurs. 

Most known thiamin diphosphate-dependent reactions (Table 14-2) can be derived from the five halfreactions, a through e, shown in Fig. 14-3. Each half-reaction is an a cleavage which leads to a thiamin- bound enamine (center. Fig. 14-3) The decarboxylation of an a-oxo acid to an aldehyde is represented by step h followed by fl in reverse. The most studied enzyme catalyzing a reaction of this type is yeast pyruvate decarboxylase, an enzyme essential to alcoholic fermentation (Fig. 10-3). There are two 250-kDa isoenzyme forms, one an tetramer and one with an (aP)2 quaternary structure. The isolation of a-hydroxyethylthiamin diphosphate from reaction mixtures of this enzyme with pyruvate provided important verification of the mechanisms of Eqs. 14-14,14-15. Other decarboxylases produce aldehydes in specialized metabolic pathways indolepyruvate decarboxylase in the biosynthesis of the plant hormone indole-3-acetate and ben-zoylformate decarboxylase in the mandelate pathway of bacterial metabolism (Chapter 25). Formation of a-ketols from a-oxo acids also starts with step h of Fig. 14-3 but is followed by condensation with another carbonyl compound in step c, in reverse. An example is decarboxylation of pyruvate and condensation of the resulting active acetaldehyde with a second pyruvate molecule to give l -a-acetolactate, a reaction catalyzed by acetohydroxy acid synthase (acetolactate synthase). Acetolactate is the precursor to valine and leucine. A similar ketol condensation, which is catalyzed by the same S5mthase, is... 

Enantiomerically pure dipeptide is obtained when the 4-nitrophenyl ester of N-benzoyl-L-leucine is coupled with ethyl glycinate in ethyl acetate. If, however, the leucine ester is treated with 1-methylpiperidine in chloroform for 30 min prior to coupling, the dipeptide in nearly completely racemized. Treatment of the leucine ester with 1-methylpiperidine leads to formation of a crystalline material of composition C13H15NO2, which has strong IR bands at 1832 and 1664 cm Explain how racemization occurs and suggest a reasonable structure for the crystalline material. 

Isotope-labeled studies showed that BAR is biosynthetically derived from L-leucine, L-cysteine, L-phenylalanine, acetate, and SAM. The biosynthetic pathway starts from L-leucine, which by chlorination leads to trichloroleucine, a direct precursor for BAR biosynthesis [370],... 

Several reactions of amino acids result in heterocyclic derivatives. In one example, the carbonyl unit of leucine (55) is converted to its iV-acetyl derivative (94). Subsequent treatment with acetic anhydride and sodium acetate leads to a heterocycle, 5-oxazolone, 95. An oxazolone derived from amino acids has the common name ofazlactone, so 95 is the azlactone of leucine. 

Another amino acid synthesis is called the azlactone synthesis. Remember from before that an azlactone is an oxazolone (see 95). When glycine (52) is converted to its AT-benzoyl derivative (112 known as hippuric acid) by reaction with benzoyl chloride, treatment with acetic anhydride (AC2O) gives the azlactone 113. This is the reaction presented in the preceding section (see compormd 95). Compound 110 has the common name of hippuric acid azlactone. As with the thiohydantoin, the -CH2- unit in 113 is susceptible to an enolate anion condensation reaction with aldehydes (Chapter 22, Section 22.7.2), and reaction with 2-methylpropanal in the presence of pyridine gives azlactone 114. Catalytic hydrogenation of the alkene unit (Chapter 19, Section 19.3.2) and acid hydrolysis lead to the amino acid leucine (55). 

Advances in the knowledge of the metabolic reactions of acetoacetate and the further studies of Coon on leucine metabolism solved the remaining uncertainties in the interpretation of the previous experimental data. This author demonstrated that the 3 carbons of the isopropyl group of leucine are incorporated as a unit into the 2-, 3-, and 4-positions of aceto-acetic acid. The carboxyl carbon probably arises by a CO2 fixation reaction, which had not been recognized previously. According to this scheme the complete breakdown of a mole of leucine by liver leads to the formation of approximately 1.5 moles of ketone bodies. 

Branched-chain amino acids as precursurs of plant isoprenoids L-Leucine and L-valine, when fed to germinating pea seeds (Pisum sativum) were incorporated into squalene and 6-amyrin [39]. Chemical degradation by ozonolysis of the radioactive squalene revealed an equal distribution of the radioactivity in the IPP-derived (4 portions per molecule) and the DMAPP-derived moieties (2 portions per molecule). However, an unbalanced distribution in favor of the DMAPP-derived moiety of monoterpenoids was found after feeding radioisotopically labeled L-leucine, L-valine, DL-alanine, acetate, and R,S-MVA to intact plants of Cinnamomum camphora and of Pelargonium roseum [40]. The relatively low incorporation of amino acids was explained by several hypotheses a) the radioactivity of amino acids is scattered into other metabolites rather than monoterpenoids b) there is a great pool of the amino acids which leads to dilution of radioactivity, and c) the low permeation of the amino acids into the biosynthetic site of monoterpenoids [40]. The DMAPP-derived moiety of monoterpenoids, biosynthesized from [U- C]leucine and [U- ]valine was labeled with more than 64% of the incorporated tracers, while this moiety when derived from [2- " C]MVA contained less than 32% of the tracers [40]. This lends support to the interpretation that both amino acids are incorporated not via MV A, but by an alternate route, e,g, a combination of the well known mammalian leucine degradation pathway and a reversal of the MVA shunt (see below). The distribution pattern in monoterpenoids after administration of [2- ]alanine was similar to that after incorporation of MVA, which indicates that alanine is first metabolized to acetyl-CoA, which then constructs preferentially the IPP-derived moiety of the monoterpenoids via MVA [40]. 

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