Radiochemical assays for p-hydroxyphenylpyruvate hydroxylase activity in human liver

In vivo fluorescent screening for HPPD-targeted herbicide discovery

BACKGROUND

4-Hydroxyphenylpyruvate dioxygenase (HPPD), playing a critical role in vitamin E and plastoquinone biosynthesis in plants, has been recognized as one of the most important targets for herbicide discovery for over 30 years. Structure-based rational design of HPPD inhibitors has received more and more research interest. However, a critical challenge in the discovery of new HPPD inhibitors is the common inconsistency between molecular-level HPPD-based bioevaluation and the weed control efficiency in fields, due to the unpredictable biological processes of absorption, distribution, metabolism, and excretion.

RESULTS

In this study, we developed a fluorescent-sensing platform of efficient in vivo screening for HPPD-targeted herbicide discovery. The refined sensor has good capability of in situ real-time fluorescence imaging of HPPD in living cells and zebrafish. More importantly, it enabled the direct visible monitoring of HPPD inhibition in plants in a real-time manner.

CONCLUSION

We developed a highly efficient in vivo fluorescent screening method for HPPD-targeted herbicide discovery. This discovery not only offers a promising tool to advance HPPD-targeted herbicide discovery, but it also demonstrates a general path to develop the highly efficient, target-based, in vivo screening for pesticide discovery. © 2022 Society of Chemical Industry.

4-Hydroxyphenylpyruvate Dioxygenase Inhibitors: From Chemical Biology to Agrochemicals

The development of new herbicides is receiving considerable attention to control weed biotypes resistant to current herbicides. Consequently, new enzymes are always desired as targets for herbicide discovery. 4-Hydroxyphenylpyruvate dioxygenase (HPPD, EC 1.13.11.27) is an enzyme engaged in photosynthetic activity and catalyzes the transformation of 4-hydroxyphenylpyruvic acid (HPPA) into homogentisic acid (HGA). HPPD inhibitors constitute a promising area of discovery and development of innovative herbicides with some advantages, including excellent crop selectivity, low application rates, and broad-spectrum weed control. HPPD inhibitors have been investigated for agrochemical interests, and some of them have already been commercialized as herbicides. In this review, we mainly focus on the chemical biology of HPPD, discovery of new potential inhibitors, and strategies for engineering transgenic crops resistant to current HPPD-inhibiting herbicides. The conclusion raises some relevant gaps for future research directions.

SC-0051, a 2-benzoyl-cyclohexane-1,3-dione bleaching herbicide, is a potent inhibitor of the enzyme p-hydroxyphenylpyruvate dioxygenase

Growth inhibition of Lemna gibba plantlets by the bleaching herbicide, SC-0051 (2-(2-chloro-4-methanesulfonylbenzoyl)-1,3-cyclohexanedione)) was alleviated by the addition of homogentisic acid to the growth medium. Homogentisic acid is a key intermediate in the biosynthesis of tyrosine-derived plant quinones as well as in tyrosine metabolism. The herbicide prevented the incorporation of radioactivity from [14C]tyrosine into lipophilic plant metabolites and, in rat liver extracts, the herbicide inhibited the conversion of tyrosine to homogentisic acid. The enzyme p-hydroxyphenylpyruvate dioxygenase (EC 1.13.11.27) from both Zea mays seedlings and liver tissues, was found to be subject to strong inhibition by SC-0051. Inhibition of plant quinone biosynthesis is a new mode of herbicidal action. One of the consequences of quinone depletion in plants by SC-0051. Inhibition of plant quinone biosynthesis is a new mode of herbicidal action. One of the consequences of quinone depletion in plants in vivo is apparently an indirect inhibition of phytoene desaturation. The enzyme phytoene desaturase itself, however, is not afflicted by the herbicide.

Biochemical studies on the enzymatic deficiencies in hereditary tyrosinemia

Experiments are described on the effects of succinylacetone and fumarylacetoacetate on delta-aminolevulinic acid dehydratase, methionine adenosyltransferase and p-OH-phenylpyruvate dioxygenase. delta-Aminolevulinic acid dehydratase from human erythrocytes is inhibited non-competitively by succinylacetone (Ki 0.03 mumol/l) and by fumarylacetoacetate (Ki 0.06 mumol/l). The inhibition by succinylacetone is not prevented by dithiothreitol, but the inhibition by fumarylacetoacetate is not observed if dithiothreitol is present. Methionine adenosyltransferase, partially purified from rabbit liver, is not inhibited by succinylacetone but is inhibited by fumarylacetoacetate: 69% inhibition is observed at 1 mmol/l. Human liver p-OH-phenylpyruvate dioxygenase is not inhibited by succinylacetone or fumarylacetoacetate. It is concluded that secondary enzyme deficiencies observed in hereditary tyrosinemia (delta-aminolevulinic acid dehydratase, methionine adenosyl transferase) are the result of inhibition by succinylacetone and fumarylacetoacetate, accumulating as a result of a primary deficiency of fumarylacetoacetase.

Some kinetic properties of 4-hydroxyphenylpyruvate dioxygenase from Pseudomonas sp. strain P.J. 874

Steady-state kinetic properties of purified 4-hydroxyphenylpyruvate dioxygenase from Pseudomonas sp. strain P.J. 874 were examined by a 14CO2 method at pH 7.5 and 37 degrees C. The results conform to a mono-iso-ordered bi-bi mechanism with binding of 4-hydroxyphenylpyruvate before O2 and release of CO2 before homogentisate. A Theorell-Chance mechanism can not be excluded. The apparent DV and DV/Km ratios were about 1.1 and 1.2, respectively, for 4-hydroxy[2,6-2H2]phenylpyruvate when the initial O2 consumption was measured. The stoichiometry between the consumption of O2 and the formation of homogentisate and CO2 was 1:1:1. Loss of hydrogen at C2 or C6 of the ring of the substrate is thus not the rate-limiting step during the reaction. Instead, this appears to be the conversion of an isomerized enzyme form.

The formation of homogentisate in the biosynthesis of tocopherol and plastoquinone in spinach chloroplasts

Homogentisate is the precursor in the biosynthesis of α-tocopherol and plastoquinone-9 in chloroplasts. It is formed of 4-hydroxyphenylpyruvate of the shikimate pathway by the 4-hydroxyphenylpyruvate dioxygenase. In experiments with spinach the dioxygenase was shown to be localized predominatedly in the chloroplasts. Envelope membranes exhibit the highest specific activity, however, because of the high stromal portion of chloroplasts, 60-80% of the total activity is housed in the stroma. The incorporation of 4-hydroxyphenylpyruvate into 2-methyl-6-phytylquinol as the first intermediate in the tocopherol synthesis by the two-step-reaction: 4-Hydroxyphenylpyruvate → Homogentisate[Formula: see text] 2-Methyl-6-phytylquinol was demonstrated by using envelope membranes. Homogentisate originates directly from 4-hydroxyphenylpyruvate of the shikimate pathway. Additionally, a bypass exists in chloroplasts which forms 4-hydroxyphenylpyruvate from tyrosine by an L-amino-acid oxidase of the thylakoids and in peroxisomes by a transaminase reaction. Former results about the dioxygenase in peroxisomes were verified.

Tritium isotope effects in the reaction catalyzed by 4-hydroxyphenylpyruvate dioxygenase from pseudomonas sp. strain P.J. 874

Tritium isotope effects in the reaction catalyzed by 4-hydroxyphenylpyruvate dioxygenase (4-hydroxyphenyl-pyruvate:oxygen oxidoreductase (hydroxylating, decarboxylating), EC 1.13.11.27) from Pseudomonas sp. strain P.J. 874 were studied with 14C- and different 3H-labelled 4-hydroxyphenylpyruvate. Tritium of ring-2,6-3H2-labelled substrate was released into water in 1:2 stoichiometry to 14CO2 formation. The tritium release from ring-3,5-3H2- and side chain-3-3H1-labelled 4-hydroxyphenylpyruvate was low as compared with 14CO2 formation. The apparent tritium isotope effects were below two, as judged by comparison of 3H/14C ratios of 4-hydroxyphenylpyruvate and homogentisate. The ratios showed no dependence on oxygen concentrations between 1 and 21% in the gas phase. Thus, a tritium assay can be used to determine the activity of 4-hydroxyphenylpyruvate dioxygenase. Apparently, none of the substrate hydrogens is involved in any rate-limiting step up to the first irreversible step. enol-4-Hydroxyphenylpyruvate was excluded as the active substrate tautomer.

Inhibition of 4-hydroxyphenylpyruvate dioxygenase from Pseudomonas sp. strain P.J. 874 the enol tautomer of the substrate

Progressive inactivation of purified 4-hydroxyphenylpyruvate dioxygenase (4-hydroxyphenylpyruvate:oxygen oxidoreductase (hydroxylating, decarboxylating), EC 1.13.11.27) from Pseudomonas sp. strain P.J. 874 by enol-4-hydroxyphenylpyruvate was initially pseudo-first-order with respect to the remaining enzymic activity, as measured with an enol-borat assay at pH 7.5 and 37 degrees C. No inhibitory product was detected. Saturation kinetics suggests formation of a reversible complex prior to an inactivation event at the active site of the enzyme. The initial concentration of enol-4-hydroxyphenylpyruvate, which gave half-maximum inactivation, varied linearly with the assay concentration of ascorbate from 30 microM at zero (extrapolated value) to 0.8 mM at 20 mM ascorbate. The limiting rate constant for the inactivation increased linearly from 0.01 to 0.02 s-1 in this interval. Inhibition by ascorbate present during preincubations was partially relieved by enol-4-hydroxyphenylpyruvate. Inhibition by 1,2-dihydroxybenzene-3,5-disulfonic acid present during preincubations was prevented by ascorbate but not reversed by enol-4-hydroxyphenylpyruvate. The reductively-activated enzyme used keto-4-hydroxyphenylpyruvate as substrate for formation of 14CO2 and homogentisate. enol-4-Hydroxyphenylpyruvate was a noncompetitive inhibitor vs. keto-4-hydroxyphenylpyruvate with an intercept inhibition constant of about 40 microM when a 14CO2 assay was used. It is suggested that interaction of enol-4-hydroxyphenylpyruvate with enzyme-bound Fe3+, formed by autooxidation, caused the substrate inhibition of 4-hydroxyphenylpyruvate dioxygenase, long known to be relieved by a variety of reductants. The possible role for the inhibition mechanism in the regulation of tyrosine catabolism in vivo is discussed.

On 4-hydroxyphenylpyruvate dioxygenase of adult frog liver

1. It has been reported that 4-hydroxyphenylpyruvate dioxygenase (EC 1.13.11.27) activity in the liver from Rana esculenta is present only after autolysis of trypsin digestion, which releases a heat-and acid-stable inhibitor of low molecular mass. 2. Attempts to demonstrate similar effects with the liver enzyme from adult Rana pipiens were unsuccessful. Trypsin had only an inhibitory effect on the enzyme activity in crude extracts. 3. Both untreated and trypsin-treated enzyme had a molecular mass of about 100,000 daltons as determined by gel filtration. The pI was around pH 4.6. One pH-optimum between pH 7 and 8 was observed. 4. At pH 7.5 and 37 degrees C the basal enzyme activity was 1.3 mumol/min per g of protein. It was increased six-fold by a reductant in the presence of catalase. Fe2+ (50 muM) increased the activity further 1.6-fold when the reaction was carried out in Tris-HCl buffer, but not in potassium phosphate buffer. 5. The Km for 4-hydroxyphenylpyruvate was 50 muM and the Vmax was around 10 mumol/min per g of soluble protein with reductively activated enzyme. 6. Substrate inhibition was observed above 20 muM concentrations of 4-hydroxyphenylpyruvate.

On the renal tubular damage in hereditary tyrosinemia and on the formation of succinylacetoacetate and succinylacetone

Phenylalanine and homogentisate increase the concentration of succinylacetoacetate and succinylacetone both in serum and urine in patients with hereditary tyrosinemia and therefore increase the excretion of 5-aminolevulinate. Both phenylalanine and homogentisate cause a tubular proteinuria which is in agreement with our hypothesis that their metabolites maleylacetoacetate and fumarylacetoacetate are the toxic compounds in hereditary tyrosinemia. The patient with the highest excretion of succinylacetoacetate and succinylacetone has the slightest tubular proteinuria whereas the one with the lowest excretion of these compounds has the more pronounced tubular proteinuria. It is suggested that this is caused by a difference in the ability to reduce the presumed toxic compounds fumarylacetoacetate and maleylacetoacetate, i.e. the precursors of succinylacetoacetate.

TPN and Mn-isocitrate protect isocitrate dehydrogenase against inactivation but increase the number of modified sulfhydryl groups

In a pilot group of 20 sequential patients who underwent metrizamide lumbar myelography, three modifications in technique were compared: (1) a “30” min delayed frontal projection, (2) supine projection of the conus medullaris, and (3) horizontal beam oblique views of the lumbosacral theca. The study showed much better opacification of the lowermost lumbar and the sacral root sleeves by delayed filming in most (70%) of the cases; that the conus medullaris, a structure difficult to visualize by the routine method of prone positioning, could be well visualized routinely with the patient supine; and that larger numbers of lumbar nerve root sleeves could be seen with the same degree of table tilt on horizontal beam oblique than on vertical beam oblique radiography.