Tyrosine, labelled with

Isotopic experiments shed more light on the contradictory evidence. Weinhouse and Millington (905) incubated liver slices with tyrosine labeled with in the 8-position of the side chain, and found that the activity appeared in the methylene carbon of acetoacetate. Schepartz and Gurin (772, 773) incubated with liver slices phenylalanine labeled with in the carboxyl group or a-position, or in positions 1, 3, and 5 of the aromatic ring. They found that the a-carbon of the side chain became the carboxyl-carbon of acetoacetate either C-1 or C-3 of the ring became the terminal methyl... 

In contrast with phenylalanine, tyrosine labeled with C in the ring is readily converted to by plant cells. Using tissue cultures from 10... 

This reaction was studied by use of tritium. The phenylalanine was labeled with tritium at the 4-position of the phenyl ring. When the product, tyrosine, was isolated, it retained much of the original radioactivity, even though the 4-position was now substituted by a... 

The thiazolecarboxylic acid structure (40) was also guessed in a similar way, from tracer experiments. The unknown compound was converted into the thiamine thiazole by heating at 100°C and pH 2. On paper electrophoresis, it migrated as an anion at pH 4. Tracer experiments indicated that it incorporated C-l and C-2 of L-tyrosine, and the sulfur of sulfate. The synthetic acid was prepared by carboxylation of the lithium derivative of the thiamine thiazole, and the derivatives shown in Scheme 19 were obtained by conventional methods. Again, the radioactivity of the unknown, labeled with 35S could not be separated from structure 40, added as carrier, and the molar radioactivity remained constant through several recrystallizations and the derivatizations of Scheme 17. 

The liver is also the principal metabolic center for hydrophobic amino acids, and hence changes in plasma concentrations or metabolism of these molecules is a good measure of the functional capacity of the liver. Two of the commonly used aromatic amino acids are phenylalanine and tyrosine, which are primarily metabolized by cytosolic enzymes in the liver [1,114-117]. Hydroxylation of phenylalanine to tyrosine by phenylalanine hydroxylase is very efficient by the liver first pass effect. In normal functioning liver, conversion of tyrosine to 4-hy-droxyphenylpyruvate by tyrosine transaminase and subsequent biotransformation to homogentisic acidby 4-hydroxyphenylpyruvic acid dioxygenase liberates CO2 from the C-1 position of the parent amino acid (Fig. 5) [1,118]. Thus, the C-1 position of phenylalanine or tyrosine is typically labeled with and the expired C02 is proportional to the metabolic activity of liver cytosolic enzymes, which corresponds to functional hepatic reserve. Oral or intravenous administration of the amino acids is possible [115]. This method is amenable to the continuous hepatic function measurement approach by monitoring changes in the spectral properties of tyrosine pre- and post-administration of the marker. 

Tritiated digoxin has been used as a tracer in most of the published procedures, but recently techniques employing [ I]tyrosine-labeled digoxin have been described. Such techniques not only are more sensitive, but have the added advantage of dispensing with the necessity to use scintillation counting. 

The naturally occurring aromatic amino acids phenylalanine, tryptophane and tyrosine (Fig. 1) have been labelled with fluorine-18 through similar electrophilic substitution methods [7]. Aromatic residues contained in peptides have been labelled with CH3C02[ F]F [105,106], an example of direct labelling of macromolecules. However, direct labelling of macromolecules is usually not the method of choice nowadays (see Section 6). 

A more extensive study of mobilities of 3H- and 14C-labeled amino acids again found that amino acids labeled with 14C at Cl or C2 are retained on the column, relative to the unlabeled forms.135 Lysine is an exception. Tritiation at C3 also increases the retention time, but tritiation at C2 of glycine or at C4, C5, or C6 of lysine decreases it, and large decreases are seen with methionine tritium-labeled in the methyl and with tyrosine tritium-labeled at C3, 5. The 14C IEs can be attributed to a decrease of acidity, but the IEs of distant 3H may be due to hydrophobic interactions with the resin. A remarkable result is that intramolecular isotopic isomers (isotopomers) can be distinguished on the basis of their chromatographic mobilities. 

The alternative approach to the determination of the stereochemistry of hydroxylation )3 to the nitrogen occurring in the biosynthesis of haemanthamine involved the synthesis of stereospecifically labeled tyrosine. Catalytic hydrogenation of suitable acylaminocinnamic acids labeled with isotopic hydrogen in fi position proceeds stereoselectively to furnish an equimolecular mixture of L-(/3i -3H]-and D-[j8 -3H]tyrosine (419) and (420). Enzymic resolution of the racemic amino acid yielded the L and d isomers. The two optically active forms of tyrosine were... 

The stereochemistry of 43 was established to be S by comparing the optical rotation of an authentic sample derived from (R)-//-tyrosine hydrochloride with //-tyrosine hydrochloride isolated from edeine A and B [62], Incubation experiments with [lsN]tyrosine revealed that the [15N]amino group in //-tyrosine is lost, suggesting a reaction mechanism similar to that of an ammonia lyase [62]. This finding is supported by further incubation experiments with (2RS,3R)-[3-3H]tyrosine and (2RS,3S)-[ 3-3H tyrosine in combination with (2RS)-[3-14C]tyrosine. Determination of the 3H/14C ratios of the isolated //-tyrosine 43 leads to the conclusion that the pro-3 S hydrogen is lost in the course of the reaction whereas the pro-3R hydrogen is retained (Scheme 1.6.14). A similar incubation experiment with (2S)-[2-3H]tyrosine proceeds with a loss of most of the tritium label from C2. 

Cephalotaxus Alkaloids.—Preliminary results indicate that the homo-Erythrina alkaloid schelhammeridine (52) derives from phenylalanine and tyrosine by way of a phenethylisoquinoline precursor [as (53)].52 Previous evidence for the biosynthesis of the related alkaloid cephalotaxine (54), obtained with tyrosine labelled in the side-chain, has indicated a different pathway which involves two molecules of this amino-acid.53 Recently, however, tyrosine labelled in the aromatic ring was examined as a cephalotaxine precursor and was found54 to label ring A of (54) almost exclusively, i.e. only one unit of tyrosine is used for biosynthesis. This is obviously inconsistent with the previous evidence and the early incorporations are... 

In mitochondrial research the phosphate-transporting protein from rat liver mitochondria has been labeled with ° Hg-mersalyl At For protein labeling with astatine (alpha emitter) the following procedures may be u reaction of the protein with p-astatobenzoic acid < ndensation reaction with peptide bond and protein acetylation While labeling by the above procedures seems to be sufficiently stable a remarkable instability of the At-label was obserwd when astatinated protein was prepared electrophoretically 202) jjjg results of these authors indicate that the tyrosine-astatine bond is unstable. The conclusion of Vau an et al. that astatinated proteins lore as much as 50% of their biological activity and, in addition, are extremely toxic, is very serious. 

Studies in solution indicate that the conformations of tyrosine-248 in the dissolved and crystalline states may be quite different. This conclusion is based on the work of Johansen and Vallee. They have examined the absorption and circular dichroic spectral properties of a carboxy-peptidase A derivative in which tyrosyl-248 is selectively labeled with the conformational probe, diazotized arsanilic acid 62, 63),... 

To characterize protein-ligand interactions by F NMR, the ligand or the protein can be labeled with fluorine to produce spectra without overwhelming complexity. Proteins expressed in E.coli and tissue culture have been labeled with fluorine by biosynthetic incorporation of fluoro analogs of tryptophan, phenylalanine and tyrosine. Conformational properties of receptor... 

Fig. 5 Nasal absorption of a series of diiodo-L-tyrosine-labeled dextrans with molecular masses of 1260-45,500 Da in rats. (Reprinted from Ref. " with permission from Elsevier.)...

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