Lipoamidase (lipoyl-X hydrolase) from pig brain

Mitochondrial fatty acid synthesis and respiration

Recent studies have revealed that mitochondria are able to synthesize fatty acids in a malonyl-CoA/acyl carrier protein (ACP)-dependent manner. This pathway resembles bacterial fatty acid synthesis (FAS) type II, which uses discrete, nuclearly encoded proteins. Experimental evidence, obtained mainly through using yeast as a model system, indicates that this pathway is essential for mitochondrial respiratory function. Curiously, the deficiency in mitochondrial FAS cannot be complemented by inclusion of fatty acids in the culture medium or by products of the cytosolic FAS complex. Defects in mitochondrial FAS in yeast result in the inability to grow on nonfermentable carbon sources, the loss of mitochondrial cytochromes a/a3 and b, mitochondrial RNA processing defects, and loss of cellular lipoic acid. Eukaryotic FAS II generates octanoyl-ACP, a substrate for mitochondrial lipoic acid synthase. Endogenous lipoic acid synthesis challenges the hypothesis that lipoic acid can be provided as an exogenously supplied vitamin. Purified eukaryotic FAS II enzymes are catalytically active in vitro using substrates with an acyl chain length of up to 16 carbon atoms. However, with the exception of 3-hydroxymyristoyl-ACP, a component of respiratory complex I in higher eukaryotes, the fate of long-chain fatty acids synthesized by the mitochondrial FAS pathway remains an enigma. The linkage of FAS II genes to published animal models for human disease supports the hypothesis that mitochondrial FAS dysfunction leads to the development of disorders in mammals.

Determination of specific activities and kinetic constants of biotinidase and lipoamidase in LEW rat and Lactobacillus casei (Shirota)

Enzyme kinetic parameters, such as K(m), V(max) (or V), k(cat)/K(m), and K(i) (by biotin or lipoic acid) for biotinidase and lipoamidase were determined in Lewis (LEW) rat and Lactobacillus casei (Shirota) using fluorimetric high-performance liquid chromatography (HPLC). It was found that the final protein concentration below 0.1mg/ml is sufficient to obtain linear hydrolytic reaction and to determine the Michaelis-Menten type kinetic parameters (K(m), V, K(i)). We applied this HPLC enzyme assay method onto the rat and some bacteria. The highest specific activities (Vs) for biotinidase were found in Lactobacillus casei (Shirota) and rat kidney. It was also found that the largest K(i) by product for biotinidase and lipoamidase were present in the Lactobacillus casei (Shirota). There has been found specie (between rat and mouse) differences and tissue (organ) differences, together with tissue region differences and sex differences in some tissues. Summary of the distributions of both enzymes in LEW rat was also presented. Therefore, this HPLC determination method for the enzyme kinetic parameters in tissues is expected to be an indispensable tool for the investigation of the various diseases in humans.

Assay of protein-bound lipoic acid in tissues by a new enzymatic method

A new enzymatic method for the determination of protein-bound lipoic acid was established. Bound lipoyl groups were liberated in the form of lipoyllysine by protease digestion and assayed by lipoamide dehydrogenase (NADH:lipoamide oxidoreductase, EC 1.8.1.4)-mediated NADH oxidation. NADH oxidation was coupled to reduction of the lipoyl disulfide group. Fluorescence kinetics of NADH oxidation were markedly enhanced by the addition of glutathione disulfide, recycling the enzyme-mediated lipoyl/dihydrolipoyl conversion. In the presence of a large excess of glutathione disulfide, NADH oxidation follows pseudo-first-order kinetics in terms of lipoyllysine concentration. A good linear correlation is obtained between the oxidation rate and lipoyllysine concentration up to 5 microM and the calibration curve indicates that the detection limit could be 100 nM lipoyllysine. The method was applied to protease lysates of bovine, rat, and rabbit tissues to determine lipoyllysine levels. Kidney and liver were found to have the highest content of lipoyllysine in the range of 3.9 to 4.6 nmol/g rat or rabbit wet tissue or 11.6 to 13.1 nmol/g bovine acetone powder.

High-performance liquid chromatographic determination of lipoamidase (lipoyl-X hydrolase) activity with a novel substrate, lipoyl-6-aminoquinoline

An HPLC lipoamidase (lipoyl-X hydrolase) assay method has been developed, which uses a novel fluorescent substrate, lipoyl-6-aminoquinoline (LAQ). LAQ is synthesized from lipoic acid and 6-aminoquinoline (AQ) through lipoyl chloride as an intermediate and is conveniently purified by washing with chloroform-methanol. Mechanistic studies on the time-course, the dependence on enzyme and substrate concentrations were performed by using LAQ and a model enzyme (milk lipoamidase). Moreover, this method was successfully applied to the direct determination of the lipoamidase (LAQ hydrolase) activity in samples of human liver, milk, stools and porcine serum. Using this novel synthetic lipoyl substrate, we demonstrated that LAQ hydrolase was present in some specific tissues; LAQ hydrolase was solely present in the grey matter and not in the white matter in the human cerebrum. Furthermore, LAQ hydrolase activity was shown to increase in human liver cancer. Thus, this enzyme assay method is expected to be applicable to the tissue distribution study and also to the basic research on human diseases such as cancer.

Characterization of the kinetics and distribution of N-arachidonylethanolamine (anandamide) hydrolysis by rat brain

Arachidonoylethanolamide or 'anandamide' is a naturally occurring derivative of arachidonic acid that has been shown to activate cannabinoid receptors in the brain. Its metabolic inactivation by brain tissue has been investigated. Anandamide is hydrolyzed by the membrane fraction of rat brain homogenate to arachidonic acid and ethanolamine. The hydrolysis is temperature and pH- dependent (pH maximum at 8.5) and abolished by boiling. Anandamide hydrolysis is protein dependent in the range of 25-100 micrograms protein/ml; does not require calcium and is inhibited by phenylmethylsulfonylfluoride, diisopropylfluorophosphate, thimerosal and arachidonic acid. Hydrolysis of 10 microM anandamide by brain membranes follows first order kinetics; at 30 degrees C, the rate constant for anandamide catabolism is 0.34 min-1 mg protein-1. The Km for anandamide hydrolysis is 3.4 microM, and the Vmax is 2.2 nmol/min per mg protein. Hydrolysis occurs in all subcellular fractions except cytosol with the highest specific activity in myelin and microsomes. The distribution of anandamide hydrolytic activity correlates with the distribution of cannabinoid receptor-binding sites; the hippocampus, cerebellum and cerebral cortex exhibit the highest metabolic activity, while activity is lowest in the striatum, brain stem and white matter.

Lipoamidase and biotinidase activities in the rat: tissue distribution and intracellular localization

Lipoamidase (not yet given an EC number) activity was measured in various rat tissues using two different substrates, one natural, lipoyllysine (epsilon-N-(D,L-lipoyl)L-lysine) and one artificial, lipoyl-p-aminobenzoic acid (N-D,L-lipoyl-p-aminobenzoic acid). Biotinidase, EC 3.5.1.12, was measured in the same tissue with the artificial substrate, biotinyl-p-aminobenzoic acid (N-D-biotinyl-p-aminobenzoic acid). Lipoamidase measured as lipoyl-p-aminobenzoic acid hydrolase activity had two pH optima, at pH 6.0 and pH 9.5, in liver homogenate, but only one pH optimum at pH 6.0 in rat plasma. Lipoamidase measured as lipoyllysine hydrolase activity had a pH optimum at pH 5.5 both in liver homogenate and plasma. Similarly, biotinidase shows a single pH optimum at pH 6.0 in liver homogenate and plasma. The properties of lipoyllysine hydrolase and biotinidase were similar with respect to thermostability, pH stability and inhibition pattern, and their properties differed from those of lipoyl-p-aminobenzoic acid hydrolase. Lipoyllysine hydrolase and biotinidase activities were highest in kidney, liver and blood plasma, whereas lipoyl-p-aminobenzoic acid hydrolase activities were highest in liver, brain and kidney. Lipoyllysine hydrolase and biotinidase activities were found mainly in the liver microsomal fraction, and lipoyl-p-aminobenzoic acid hydrolase was recovered from the microsomal fraction and to a small extent from the mitochondrial fraction. These results indicate that liver lipoyl-p-aminobenzoic acid hydrolase is an enzyme protein which differs from lipoyllysine hydrolase, and the data also indicate that liver lipoyllysine hydrolase and biotinidase are the same enzyme protein.

High-recovery protein purification by high-performance gel-permeation chromatography: application to human serum biotinidase

A reproducible, high-yield high-performance gel-permeation chromatographic method for proteins was developed and applied to the final step of purification of human serum biotinidase. One diol-type silica gel column, Tosoh TSK-gel 3000 SW (300 mm x 7.9 mm I.D.; average pore size 30 nm), and two Jasco Bio-Fine GFC SI 150-K columns (300 mm x 7.9 mm I.D.; average pore size 15 nm) were connected in series. A 0.1 M sodium phosphate buffer (pH 6.0) solution containing 0.3 M sodium chloride, glycerol (2.5%, v/v) and the non-ionic detergent Nonidet P-40 (NP-40, 0.15%, v/v) was utilized as an eluent. Recovery of the protein bovine serum albumin from the separating columns was measured by a spectrophotometric method and found to be 77.0 +/- 3.74% (mean +/- S.D.). The recovery of the total activity of human serum biotinidase was 72.6 +/- 13.0%. Since biotinidase activity was not recovered from the column in the absence of NP-40, the introduction of this non-ionic detergent to the mobile phase was shown to be essential for the final purification step of human serum biotinidase.

Co-purification of human serum lipoamidase and biotinidase: evidence that the two enzyme activities are due to the same enzyme protein

A more than 20000-fold purification of human serum lipoamidase is described. This was accomplished by (NH4)2SO4 precipitation and chromatography on DEAE-Sepharose, Blue Sepharose CL-6B and phenyl-Sepharose CL-4B, followed by preparative isoelectric focusing (IEF) and finally by gel-permeation chromatography. Co-precipitation and co-chromatography of lipoamidase and biotinidase activities with equal yields and purification were obtained in all the purification steps, indicating that lipoamidase and biotinidase activities in human serum are due to the same enzyme protein. After preparative IEF, two fractions with both lipoamidase activity and biotinidase activity were found at pI 4.0 and pI 4.4 respectively. The molecular mass of the enzyme was found to be 76 kDa. When 2-mercaptoethanol was used instead of cysteine as stabilizer during the purification procedure, only one major form (pI 4.0) of the enzyme was obtained after preparative IEF. By addition of cysteine, this form was transformed to an enzyme with pI 4.4, indicating that this latter form is a cysteine adduct, produced during the IEF procedure.

Lipoamidase activity in normal and mutagenized pancreatic cholesterol esterase (bile salt-stimulated lipase)

Purified human milk lipoamidase was digested with endoproteinase Lys-C and the digested peptides were subjected to gasphase microsequence analysis. The sequencing of three isolated peptides of human milk lipoamidase revealed the identity of this protein with human milk bile salt-stimulated lipase (pancreatic cholesterol esterase). The identity of the cholesterol esterase with lipoamidase was confirmed by expressing a recombinant form of rat pancreatic cholesterol esterase and testing for lipoamidase activity of the recombinant protein. The results showed that the recombinant cholesterol esterase displayed both lipolytic and lipoamidase activities and was capable of hydrolysing triacetin and lipoyl-4-aminobenzoate (LPAB). The mechanisms of the esterase and amidase activities of the enzyme were further tested by determining enzyme activity in a mutagenized cholesterol esterase with a His435-->Gln435 substitution. This mutation has been shown previously to abolish enzyme activity against esterase substrates [DiPersio, Fontaine and Hui (1991) J. Biol. Chem. 266, 4033-4036]. We showed that the mutagenized protein was effective in hydrolysing the amidase substrate LPAB and displayed similar enzyme kinetics to those of the native enzyme. These data indicate that the mechanism for the cholesterol esterase hydrolysis of lipoamides is different from that of the hydrolysis of substrates with an ester linkage. The presence of an enzyme in the gastrointestinal tract capable of both ester and amide hydrolysis suggests an important role for this protein in the digestion and absorption processes.

Release of anchored membrane enzymes by lipoamidase

Lipoamidase is found to be able to release various membrane-anchored enzymes from the membrane compartment of pig brain. Released enzymes revealed their intact enzyme activities in the soluble fraction. Lipoamidase could release at least two types of anchored enzymes, i.e. glycosyl-phosphatidylinositol-bonded and myristoylated enzymes, but not integral membrane bound enzymes. The reaction was competitively inhibited by lipoyllysine. This releasing mechanism found in the membranes may play important roles in the secretory mechanism of extracellular enzymes and also in the cellular signal-transduction system through topological changes in cellular enzymes.

Enkephalin hydrolysis by human serum biotinidase

Purified human serum biotinidase exhibited amino-exo-peptidase activity. Enkephalins and dynorphin A (less than 10-mer) seemed to be the most appropriate substrates among various physiological peptides in terms of the kcat/Km values. Similar kcat/Km values were obtained for both biocytin (biotinyllysine) and these opioid-neuropeptides. Neuro-oligo-peptides ranging from 2-mer to 18-mer were hydrolyzed. The presence of amino group at the carboxyl terminal position increased the kcat/Km value by decreasing the Km value. The results of inhibition studies using various kinds of antibiotic inhibitors, metals, and chelating agents indicated that enkephalin hydrolysis was mediated by the peptide-hydrolyzing center probably containing Zn ions. This aminopeptidase activity was uniquely inhibited by a vitamin of biocytin. The reason for the high content of biotinidase activity in serum may be related to the binary function of this enzyme; i.e., biocytin hydrolyzing amidase and enkephalin hydrolyzing aminopeptidase functions.

Lipoamidase is a multiple hydrolase

The substrate specificity of lipoamidase, purified from the pig brain membrane with lipoyl 4-aminobenzoate (LPAB) as a substrate, was extensively studied. This single polypeptide was found to hydrolyse the bonding between amide, ester and peptide compounds. However, stringent structural requirements were found in the substrates, e.g. LPAB was hydrolysed, whereas biotinyl 4-aminobenzoate was not, as stated in our previous paper [Oizmui & Hayakawa (1990) Biochem. J. 266, 427-434]. The enzyme specifically recognized the whole molecular structure of the substrate, whereas it loosely recognized the bond structure of the substrate; e.g. the dipeptide Asp-Phe was not hydrolysed, whereas the methyl ester of Asp-Phe (aspartame) was. The exopeptidase activity was demonstrated by lipoamidase; however, longer peptides than the hexamer seemed not to be substrates. Lipoyl esters, which were electrically neutral, exhibited higher specificity with longer acyl groups. Molecular mass and molecular hydrophobicity (hydropathy) seemed to determine the substrate specificity. Lipoyl-lysine, acetylcholine and oligopeptides were hydrolysed at similar Km values; however, acetylcholine was hydrolysed at a velocity 100 times higher. Although many similar specificities were found between electric eel acetylcholinesterase and lipoamidase, distinctly different specificity was demonstrated with lipoyl compounds. The role of lipoamidase, which resides on the brain membrane and possesses higher specificity for hydrophobic molecules, remains to be elucidated.