Covalent attachment of the heme prosthetic group in the CYP4F cytochrome P450 family

We demonstrated earlier that the heme in cytochrome P450 enzymes of the CYP4A family is covalently attached to the protein through an I-helix glutamic acid residue [Hoch, U., and Ortiz de Montellano, P. R. (2001) J. Biol. Chem. 276, 11339-11346]. As the critical glutamic acid residue is conserved in many members of the CYP4F class of cytochrome P450 enzymes, we investigated covalent heme binding in this family of enzymes. Chromatographic analysis indicates that the heme is covalently bound in CYP4F1 and CYP4F4, which have the required glutamic acid residue, but not in CYP4F5 and CYP4F6, which do not. Catalytic turnover of CYP4F4 with NADPH-cytochrome P450 reductase shows that the heme is covalently bound through an autocatalytic process. Analysis of the prosthetic group in the CYP4F5 G330E mutant, into which the glutamic acid has been reintroduced, shows that the heme is partially covalently bound and partially converted to noncovalently bound 5-hydroxymethylheme. The modified heme presumably arises by trapping of a 5-methyl carbocation intermediate by a water molecule. CYP4F proteins thus autocatalytically bind their heme groups covalently in a process that requires a glutamic acid both to generate a reactive (cationic) form of the heme methyl and to trap it to give the ester bond.     

Degradation of non-esterified and esterified xanthophylls by free radicals

beta-Carotene and other xanthophylls present in pepper fruit as both free and esterified forms were oxidized using a free radical initiator (2,2'-azo-bis-isobutyronitrile). Capsorubin was degraded most slowly, followed by zeaxanthin, capsanthin, and beta-carotene. The presence of keto groups at the ends of the polyene chain could be a structural factor contributing to this difference in reactivity. It was also shown that whereas capsanthin and its esters and capsorubin and its esters were degraded at the same rate, zeaxanthin esters responded differently to the oxidation process, and were degraded more quickly than free zeaxanthin. The presence of unsaturated fatty acids (mainly linoleic) that esterify zeaxanthin help to accelerate the degradation of this xanthophyll and decreasing its antioxidant action. The antioxidant capacity of capsorubin and capsanthin (both in free and esterified form) exclusive to the genus capsicum should be taken into account.     

Effects of hydrophobic and hydrophilic bile salt mixtures on cholesterol crystallization in model biles

[Ru(2,2'-bipyridine)(2)(4,4'-dicarboxy-2,2'-bipyridine)](2+) (RuBDc) is a very photostable probe that possesses favorable photophysical properties including long lifetime, high quantum yield, large Stokes' shift, and highly polarized emission. In the present study, we demonstrated the usefulness of this probe for monitoring the rotational diffusion of high-molecular-weight (MW) proteins. Using frequency-domain fluorometry with a high-intensity, blue light-emitting diode (LED) as the modulated light source, we compared the intensity and anisotropy decays of RuBDc conjugated to immunoglobulin G (IgG) and immunoglobulin M (IgM), which show a six-fold difference in MW We obtained slightly longer lifetimes for IgM (=428 ns in buffer) than IgG (=422 ns in buffer) in the absence and presence of glycerol, suggesting somewhat more efficient shielding of RuBDc from water in IgM than in IgG. The anisotropy decay data showed longer rotational correlation times for IgM (1623 and 65.7 ns in buffer) as compared to IgG (264 and 42.5 ns in buffer). Importantly, the ratio of the long rotational correlation times of IgM to IgG in buffer was 6.2, which is very close to that of MW of IgM to IgG (6.0). The shorter correlation times are most likely to be associated with domain motions within the proteins. The anisotropy decays reflect both the molecular size and shape of the immunoglobulins, as well as the viscosity. These results show that RuBDc can have numerous applications in studies of high-MW protein hydrodynamics and in fluorescence polarization immunoassays (FPI) of high-MW analytes.     

Cellular and molecular aspects of thiamin uptake by human liver cells: studies with cultured HepG2 cells

The liver is an important site for thiamin metabolism, utilization, and storage. Little is known about the mechanism of thiamin uptake by the human liver. In this study, we examined cellular and molecular aspects of the human liver thiamin uptake process using the human-derived liver HepG2 cells as a model system. Our studies showed that the initial rate of thiamin uptake to be: (1) Na(+)-independent and occurs with no detectable metabolic alterations in the transported substrate, (2) highly pH-dependent with diminished uptake upon decreasing incubation buffer pH from 8.0 to 5.0, (3) higher following cell acidification compared to unacidified control cells, (4) saturable as a function of concentration with an apparent K(m) of 7.7+/-1.6 microM, (5) inhibited by the thiamin structural analogues oxythiamin and amprolium but not by the unrelated organic cations tetraethylammonium (TEA) and N-methylnicotinamide (NMN), and (6) inhibited in a concentration-dependent manner by the membrane transport inhibitor amiloride. Both of the recently cloned human thiamin transporters, i.e., SLC19A2 and SLC19A3, were found to be expressed in liver HepG2 cells with the former being the predominant form. High promoter activity of the predominant form, i.e., SLC19A2, was detected in HepG2 cells, and the minimal region of the SLC19A2 promoter required for its basal activity in these cells was found to be encoded in a sequence between -356 and -36 and has multiple putative cis-regulatory elements. Mutation of a number of these putative cis-elements diminished promoter activity of the SLC19A2 minimal region. These results show the involvement of a specialized carrier-mediated mechanism for thiamin uptake by human liver HepG2 cells. In addition, SLC19A2 was found to be the predominant thiamin uptake carrier expressed in these cells and its promoter displays a high level of activity in them.     

Hormone-stimulated Mg(2+) accumulation into rat hepatocytes: a pathway for rapid Mg(2+) and Ca(2+) redistribution

Many diseases such as cardiac arrhythmia, diabetes, and chronic alcoholism are associated with a marked decrease of plasma and parenchymal Mg(2+), and Mg(2+) administration is routinely used therapeutically. This study uses isolated rat hepatocytes to ascertain if and under which conditions increases in extracellular Mg(2+) result in an increase in intracellular Mg(2+). In the absence of stimulation, changing extracellular Mg(2+) had no effect on total cellular Mg(2+) content. By contrast, carbachol or vasopressin administration promoted an accumulation of Mg(2+) that increased cellular Mg(2+) content by 13.2 and 11.8%, respectively, and stimulated Mg(2+) uptake was unaffected by the absence of extracellular Ca(2+). Mg(2+) efflux resulting from stimulation of alpha- or beta-adrenergic receptors operated with a Mg(2+):Ca(2+) exchange ratio of 1. These data indicate that cellular Mg(2+) uptake can occur rapidly and in large amounts, through a process distinct from Mg(2+) release, but operating only upon specific hormonal stimulation.     

The crystal structure of Mycobacterium tuberculosis alkylhydroperoxidase AhpD, a potential target for antitubercular drug design

The resistance of Mycobacterium tuberculosis to isoniazid is commonly linked to inactivation of a catalase-peroxidase, KatG, that converts isoniazid to its biologically active form. Loss of KatG is associated with elevated expression of the alkylhydroperoxidases AhpC and AhpD. AhpD has no sequence identity with AhpC or other proteins but has alkylhydroperoxidase activity and possibly additional physiological activities. The alkylhydroperoxidase activity, in the absence of KatG, provides an important antioxidant defense. We have determined the M. tuberculosis AhpD structure to a resolution of 1.9 A. The protein is a trimer in a symmetrical cloverleaf arrangement. Each subunit exhibits a new all-helical protein fold in which the two catalytic sulfhydryl groups, Cys-130 and Cys-133, are located near a central cavity in the trimer. The structure supports a mechanism for the alkylhydroperoxidase activity in which Cys-133 is deprotonated by a distant glutamic acid via the relay action of His-137 and a water molecule. The cysteine then reacts with the peroxide to give a sulfenic acid that subsequently forms a disulfide bond with Cys-130. The crystal structure of AhpD identifies a new protein fold relevant to members of this protein family in other organisms. The structural details constitute a potential platform for the design of inhibitors of potential utility as antitubercular agents and suggest that AhpD may have disulfide exchange properties of importance in other areas of M. tuberculosis biology.     

Muscle-specific expression of insulin-like growth factor I counters muscle decline in mdx mice

The Rho GTPase and Fyn tyrosine kinase have been implicated previously in positive control of keratinocyte cell-cell adhesion. Here, we show that Rho and Fyn operate along the same signaling pathway. Endogenous Rho activity increases in differentiating keratinocytes and is required for both Fyn kinase activation and increased tyrosine phosphorylation of beta- and gamma-catenin, which is associated with the establishment of keratinocyte cell-cell adhesion. Conversely, expression of constitutive active Rho is sufficient to promote cell-cell adhesion through a tyrosine kinase- and Fyn-dependent mechanism, trigger Fyn kinase activation, and induce tyrosine phosphorylation of beta- and gamma-catenin and p120ctn. The positive effects of activated Rho on cell-cell adhesion are not induced by an activated Rho mutant with defective binding to the serine/threonine PRK2/PKN kinases. Endogenous PRK2 kinase activity increases with keratinocyte differentiation, and, like activated Rho, increased PRK2 activity promotes keratinocyte cell-cell adhesion and induces tyrosine phosphorylation of beta- and gamma-catenin and Fyn kinase activation. Thus, these findings reveal a novel role of Fyn as a downstream mediator of Rho in control of keratinocyte cell-cell adhesion and implicate the PRK2 kinase, a direct Rho effector, as a link between Rho and Fyn activation.     

Glycosylation enhances functional stability of the chemotactic cytokine CCL2

The human chemokine CCL2 gene was expressed in the yeast P.pastoris and gave rise to a mixture of differently glycosylated recombinant proteins. In comparison to non-glycosylated E.coli-derived CCL2, glycosylated yeast CCL2L was 4-20 times less active in a chemotactic assay in vitro. However, CCL2L could maintain full activity upon prolonged incubation at 37 degrees C, whereas the non-glycosylated chemokine readily lost activity. It could be hypothesized that glycosylation is a mechanism used by the organism to modulate CCL2 stability. The partial loss of specific activity due to glycosylation is balanced by the advantage of prolonging the effectiveness of chemokine. Thus, differential glycosylation allows one to obtain highly effective short-lived CCL2 or less-effective long-lived CCL2 and may thus represent a novel mechanism of adaptation to pathological versus physiological conditions.     

IL-12Rbeta2-deficient mice of a genetically resistant background are susceptible to Leishmania major infection and develop a parasite-specific Th2 immune response

The cytokine IL-12 plays a critical role in inducing the production of IFN-gamma from T and NK cells and in the polarization of T cells towards the Th1 phenotype. IL-12 is comprised of two subunits (IL-12p40 and IL-12p35) that together form the biologically active p70 molecule, and IL-12 functions via binding to a heterodimeric receptor (IL-12Rbeta1 and IL-12Rbeta2). Previous studies utilizing mice deficient for either the IL-12 cytokine or the IL-12-induced signaling molecule STAT4 have established a critical role for IL-12 during infection with Leishmania major. However, these studies warrant careful re-interpretation in light of the recent discovery of the IL-12-related cytokine, IL-23, which utilizes the IL-12p40 chain in combination with an IL-12p35-related molecule, called p19, and a receptor comprised of the IL-12Rbeta1 chain plus a unique chain referred to as IL-23R. We analyzed the course of L. major infection in mice deficient for the IL-12-specific IL-12Rbeta2 subunit in order to assess the role of IL-12 signaling without disruption of the IL-23 pathway. After infection with L. major, IL-12Rbeta2KO mice of a resistant background (C57Bl/6) developed large cutaneous lesions similar to those developed by susceptible BALB/c mice. Draining lymph node cells from L. major-infected IL-12Rbeta2KO mice released the Th2 cytokines IL-4 and IL-5 after in vitro stimulation with Leishmania lysate but were completely devoid of IFN-gamma, consistent with a default towards a strong parasite-specific Th2 response. L. major-infected IL-12Rbeta2KO mice were also devoid of parasite-specific IgG2a antibodies, and interestingly, their footpad lesions ulcerated earlier than those of susceptible BALB/c mice.