New synthetic vitamin D analogs with antiproliferative activities

The introduction of oxygen atoms into different positions of the vitamin D side chain is described. By combining the arising 23-oxa and 25-oxa elements with other structural modifications (19-nor, iso-19-nor, 20-methyl, 20-ene, 20,21-cyclo) calcitriol analogs with remarkable levels of dissociation between beneficial acitivities on cell growth regulation and undesired hypercalcemia were identified. Structure-activity relations are elaborated in a very systematic outline of the Schering drug finding program in this particular class of vitamin D compounds.

Protein truncation test of LYST reveals heterogenous mutations in patients with Chediak-Higashi syndrome

Chediak-Higashi syndrome (CHS) is a rare autosomal recessive disorder in which an immune deficiency occurs in association with pigmentation abnormalities. Most patients who do not undergo bone marrow transplantation die of a lymphoproliferative syndrome, though some patients with CHS have a relatively milder clinical course of the disease. The large size of the LYST gene, defective in CHS, has made it difficult to screen for mutations in a large number of patients. Only 8 mutations have been identified so far, and all lead to a truncated LYST protein. We conducted protein truncation tests on this gene in 8 patients with CHS. Different LYST mutations were identified in all subjects through this approach, strengthening the observation of a high frequency of truncated LYST proteins as the genetic cause of CHS.

Efficient correction for CT image artifacts caused by objects extending outside the scan field of view

The purpose of this paper is to develop a method of eliminating CT image artifacts generated by objects extending outside the scan field of view, such as obese or inadequately positioned patients. CT projection data are measured only within the scan field of view and thus are abruptly discontinuous at the projection boundaries if the scanned object extends outside the scan field of view. This data discontinuity causes an artifact that consists of a bright peripheral band that obscures objects near the boundary of the scan field of view. An adaptive mathematical extrapolation scheme with low computational expense was applied to reduce the data discontinuity prior to convolution in a filtered backprojection reconstruction. Despite extended projection length, the convolution length was not increased and thus the reconstruction time was not affected. Raw projection data from ten patients whose bodies extended beyond the scan field of view were reconstructed using a conventional method and our extended reconstruction method. Limitations of the algorithm are investigated and extensions for further improvement are discussed. The images reconstructed by conventional filtered backprojection demonstrated peripheral bright-band artifacts near the boundary of the scan field of view. Images reconstructed with our technique were free of such artifacts and clearly showed the anatomy at the periphery of the scan field of view with correct attenuation values. We conclude that bright-band artifacts generated by obese patients whose bodies extend beyond the scan field of view were eliminated with our reconstruction method, which reduces boundary data discontinuity. The algorithm can be generalized to objects with inhomogeneous peripheral density and to true "Region of Interest Reconstruction" from truncated projections.

Cumulative experience with pediatric living related liver transplantation

PURPOSE

This study reports the authors' cumulative experience with pediatric living related orthotopic liver transplantation.

METHODS

The charts of all patients who received living-related liver transplantation to study complications of transplant surgery, immunosuppression, rejection, and overall survival rate were reviewed retrospectively.

RESULTS

Between November 1992 and October 1998, 30 children underwent living-related liver transplantation. Patients were between the ages of 3 months and 7 years of age (mean, 28 months). All received left lateral segmental living-related transplants. At the time of transplant, 14 of 30 patients were listed as United Network of Organ Sharing (UNOS) status 3, 11 were listed as UNOS status 2B, and 5 were listed as UNOS status 1. Indications for transplant included biliary atresia (n = 21), alpha-1-antitrypsin deficiency (n = 2), hepatitis C (n = 2), giant cell hepatitis (n = 2), hepatoblastoma (n = 1), valproic acid toxicity (n = 1), and hemangioendothelioma (n = 1). All donors were parents except for one uncle. There were no major donor complications. Minor complications included wound infection (n = 4), ventral hernia (n = 2), postoperative gastric dysmotility (n = 2), and 1 case of central line-related pneumothorax (n = 1). All but 4 recipients received primary tacrolimus immunosuppressive regimens, and the other 4 underwent conversion from cyclosporine. Initial tacrolimus therapy was begun at 0.15 mg/kg/dose PO/NG every 12 hours. Concomitant immunosuppression included methylprednisolone and mycophenolate mofetil. Fifty-three percent of patients experienced at least 1 episode of rejection, and 27% experienced multiple episodes. Immediate postoperative complications included primary nonfunction (n = 2), vascular thrombosis (n = 3), biliary leaks (n = 3), and infections (n = 17). Two patients (n = 2) required retransplantation. Complications of immunosuppressive therapy included persistent systemic hypertension (n = 6), renal tubular acidosis (n = 3), short-term hyperglycemia (n = 2), neurotoxicity (n = 2), nephrotoxicity (n = 2), food allergies (n = 8), and posttransplant lymphoproliferative disease (n = 4). All patients with PTLD were treated with immunosuppression reduction or withdrawal. Two of 4 had disease progression requiring chemotherapy. The majority of complications were treated with dose adjustments. There were 4 early deaths (13%): 1 of primary nonfunction, 2 of sepsis, and 1 of arrhythmia and renal failure. There was 1 late death of recurrent disease. Twenty-five patients (83%) are alive at 3 months to 6 years post-transplant.

CONCLUSION

Living-related orthotopic liver transplantation is an effective intervention for pediatric patients with end-stage disease.

Response of abietane diterpenes to stress in Rosmarinus officinalis L.: new insights into the function of diterpenes in plants

Abietane diterpenes were measured in field-grown rosemary (Rosmarinus officinalis L.) plants throughout the year. Carnosic acid and carnosol, which were present in high amounts (5 and 1 mg/gDW respectively) in rosemary leaves, decreased by ca. 50% during the Mediterranean summer in response to low precipitation, high radiation and high temperature. In contrast, the highly oxidised diterpenes rosmanol, isorosmanol and dimethyl isorosmanol, which are formed from carnosic acid by enzymatic dehydrogenation and the action of activated oxygen, increased in response to such environmental constraints. Collectively, these data support the contention that abietane diterpenes from R. officinalis function as antioxidants that protect biological membranes from oxidative stress. This is especially important during the Mediterranean summer when there is low precipitation, high light and high temperature.

Definition of a critical T cell threshold for prevention of GVHD after HLA non-identical PBPC transplantation in children

The aim of this study was to reduce the rate of graft failure after HLA non-identical stem cell transplantation by using G-CSF mobilized CD34+ peripheral blood progenitor cells (PBPC), either in combination with bone marrow or as single grafts. To prevent GVHD, PBPC were highly purified, resulting in a 5 to 6 log T cell depletion. In additon to T cell depletion no further GVHD prophylaxis was used. We transplanted 23 pediatric patients with life-threatening malignant or non-malignant hematological disorders, who had no available matched donor. Engraftment was obtained in 18 of 21 evaluable patients. Five patients developed acute GVHD of grade II and III, which became chronic in four cases and was fatal in four. The use of highly purified PBPC allowed the exact quantification of residual T cells in the grafts and a strict correlation between the residual T cell load and the development of GVHD was observed: patients with GVHD had received numbers of T cells between 8 and 20 x 104/kg, whereas patients without GVHD were grafted with T cell numbers ranging from 0.7 to 6.0 x 104/kg. We therefore clearly demonstrate that a residual T cell content of <5 x 104/kg is safe for prevention of GVHD after HLA non-identical PBPC transplantation in children.

Partitioning of selected antioxidants in mayonnaise

This study examined partitioning of alpha-, beta-, and gamma-tocopherol and six polar antioxidants (Trolox, ferulic acid, caffeic acid, propyl gallate, gallic acid, and catechin) in mayonnaise. Partitioning of antioxidants between different phases was determined after separation of mayonnaise by either (a) centrifugation + ultracentrifugation or (b) centrifugation + dialysis. Antioxidants partitioned in accordance with their chemical structure and polarity: Tocopherols were concentrated in the oil phase (93-96%), while the proportion of polar antioxidants in the oil phase ranged from 0% (gallic acid and catechin) to 83% (Trolox). Accordingly, proportions of 6% (Trolox) to 80% (gallic acid and catechin) were found in the aqueous phase. Similar trends were observed after dialysis. After ultracentrifugation, large proportions of polar antioxidants were found in the "emulsion phase" and the "precipitate" (7-34% and 2-7%, respectively). This indicated entrapment of antioxidants at the oil-water interface in mayonnaise. The results signify that antioxidants partitioning into different phases of real food emulsions may vary widely.

RAG and RAG defects

Products of the recombination-activating gene (RAG) play a crucial role in lymphoid cell development. During the past year, the functional properties of RAG protein domains have been better defined. Some mutations that alter the amino acid sequence of RAG1 or RAG2 have been shown to disturb B cell generation and to partially disturb T cell generation, resulting in Omenn syndrome in humans; moreover, peripheral re-expression of RAGs indicates their role in lymphoid cell homeostasis.

Antioxidant activity and partitioning of phenolic acids in bulk and emulsified methyl linoleate

To evaluate the effect of colloidal parameters on the activity of natural antioxidants, the effect of selected phenolic acids on both bulk and emulsified methyl linoleate oxidation (in the dark at 40 degrees C) was examined. Oxidation was monitored by determining the formation of hydroperoxides; their isomer distribution and ketodiene (oxodiene) products were monitored by using high-performance liquid chromatography. This study showed the system- and concentration-dependent antioxidant activity of phenolic acids. The scavenging of alpha,alpha-diphenyl-beta-picrazylhydrazyl radicals reflected the antioxidant activity in a bulk oil system but not in an emulsion. Specific interactions of the antioxidant with other compounds, for example, the emulsifier, and intramolecular hydrogen bonds may play an important role in reducing the antioxidant activity. Furthermore, these interactions of antioxidants with emulsifier have a strong influence on the partitioning of antioxidants. Thus, the proportion of the antioxidant solubilized in the lipid phase and particularly in the interface did not necessarily reflect the efficiency of the antioxidant.

Long-term complete remission and immune tolerance after intensive chemotherapy for lymphoproliferative disorders complicating liver transplant

BACKGROUND

B cell lymphoproliferative disorders (LPD) and liver rejection are major lethal complications after hepatic transplantation. Reduction in immunosuppression is the treatment for the former, but is a risk factor for the latter.

METHODS

Here, we report three consecutive children with monoclonal LPD complicating orthotopic liver transplantation. All of them were treated with brief (<4 months) but intensive chemotherapy.

RESULTS

These three patients have remained in complete remission for LPD for 18 months to more than 3 years. Aggressive antimicrobial prophylaxis was successful in preventing life-threatening infections. The patient who received the highest cumulative doses of chemotherapy may have also developed relative immune tolerance to the allograft.

CONCLUSIONS

High-dose-intensity chemotherapy may be effective in the treatment of monoclonal LPD, as well as in the induction of immune tolerance for the prevention of allograft rejection and LPD recurrence.

Structure, Chemical Bonding, and Nuclear Quadrupole Interactions of beta-Cd(OH)(2): Experiment and First Principles Calculations

The structure of Cd(OH)(2) was determined by X-ray diffraction on powder crystals and by calculations using the full-potential linearized augmented plane wave method. Good agreement between the two results was found. The chemical bonding is characterized by the interactions of the OH(-) group with Cd(2+) which is not only electrostatic but shows some polarization or covalent admixtures and by the covalent bond in the OH(-) group. The electric field gradient (EFG) was calculated and compared with an experimental determination of the nuclear quadrupole interaction using perturbed angular correlation of gamma-rays. The calculated EFG agrees well with the EFG derived from experiment. The total electric field gradient was decomposed into contributions from different orbitals and energy regions showing that both the Cd 5p and 4d wave functions contribute significantly. Finally, the influence of spin-orbit coupling on the electric field gradient was investigated and found to be of little importance.

Modulation of spontaneous and stimulation-evoked transmitter release from rat sympathetic neurons by the cognition enhancer linopirdine: insights into its mechanisms of action

The mechanisms by which the cognition enhancer linopirdine may affect transmitter release were investigated in cultures of rat superior cervical ganglion neurons. Overflow of previously incorporated [3H]noradrenaline evoked by 10 microM UTP or 0.1 microM bradykinin was enhanced by linopirdine at > or =3 microM, overflow evoked by 25 mM K(-), 100 microM nicotine, or 300 microM ATP was enhanced by linopirdine at > or =10 microM, and overflow due to 40 mM K+ or electrical field stimulation was not altered by linopirdine. Ba2+ (0.3 mM) augmented the same types of stimulation-evoked overflow to a similar extent as linopirdine. K+ (25 mM), nicotine (100 microM), and ATP (300 microM) triggered transmitter release in a partially tetrodotoxin-resistant manner, and the release-enhancing action of linopirdine was lost in the presence of tetrodotoxin (1 microM). Linopirdine (10 microM) raised spontaneous tritium outflow and reduced currents through muscarinic K+ (K(M)) channels with a similar time course. The secretagogue action of linopirdine was concentration- and Ca2(+)-dependent and abolished by tetrodotoxin (1 microM) or Cd2+ (100 microM). Linopirdine (10 microM) added to the partial inhibition of K(M) channels by 1 or 3 mM Ba2(+) but not to the complete inhibition by 10 mM Ba2(+). Likewise, the secretagogue action of 1 and 3 mM, but not that of 10 mM, Ba2+ was enhanced by linopirdine. These results indicate that linopirdine facilitates and triggers transmitter release via blockade of K(M) channels and suggest that these K+ channels are located at neuronal somata rather than at presynaptic sites.

Tissue-specific translational regulation of alternative rabbit 15-lipoxygenase mRNAs differing in their 3'-untranslated regions

By screening a rabbit reticulocyte library, an alternative 15-LOX transcript of 3.6 kb (15-LOX mRNA2) was detected containing a 1019 nt longer 3'-untranslated region (UTR2) than the main 2.6 kb mRNA (15-LOX mRNA1). In anaemic animals, northern blotting showed that 15-LOX mRNA2 was predominantly expressed in non-erythroid tissues, whereas 15-LOX mRNA1 was exclusively expressed in red blood cells and bone marrow. The 15-LOX 3'-UTR2 mRNA2 contained a novel 8-fold repetitive CU-rich motif, 23 nt in length (DICE2). This motif is related but not identical to the 10-fold repetitive differentiation control element (DICE1) of 19 nt residing in the 15-LOX UTR1 mRNA1. DICE1 was shown to interact with human hnRNP proteins E1 and K, thereby inhibiting translation. From tissues expressing the long 15-LOX mRNA2, two to three unidentified polypeptides with molecular weights of 53-55 and 90-93 kDa which bound to DICE2 were isolated by RNA affinity chromatography. A 93 kDa protein from lung cytosol, which was selected by DICE2 binding, was able to suppress translational inhibition of 15-LOX mRNA2, but not of 15-LOX mRNA1, by hnRNP E1. A possible interaction between DICE1/DICE2 cis / trans factors in translational control of 15-LOX synthesis is discussed. Furthermore, the 3'-terminal part of the highly related rabbit leukocyte-type 12-LOX gene was analysed. Very similar repetitive CU-rich elements of the type DICE1 (20 repeats) and DICE2 (nine repeats) were found in the part corresponding to the 3'-UTR of the mRNA.

The disproportion of female and male surgeons in cardiothoracic surgery

In ancient times, female medical practitioners and female surgeons were well known. With the introduction of medicine as an academic course and the ban on women studying, the medical career became virtually impossible for women. This condition changed with the general admission for women to colleges, in Germany in 1908. The current situation of women in surgery is presented here, with cardiovascular surgery as an example. Of 302 active cardiothoracic and cardiac surgeons 4.6% are female. According to an inquiry of all German heart institutes, there are 20 female senior registrars and 27 female surgeons as well as 162 female assistants. Most of the medical directors do have a positive opinion about female surgeons, but criticize that only a few women who apply for actually finish surgical training. One reason for this may be the greater difficulties for women to take care of a family and become a surgeon simultaneously. In this regard, an improvement in the position of female doctors is desirable.

Expression of leukocyte-type 12-lipoxygenase and reticulocyte-type 15-lipoxygenase in rabbits

From a rabbit reticulocyte library a full length cDNA was isolated which predicted a novel lipoxygenase (LOX) sharing 99% identical amino acids with the rabbit 15-lipoxygenase. HPLC product analysis of the bacterially expressed protein identified it as a leukocyte-type 12-lipoxygenase (1.12-LOX). This proves the co-expression of a 15-lipoxygenase and a 1.12-lipoxygenase in one mammalian species. Among the six amino acids that are different to rabbit 15-lipoxygenase, leucine 353 is shown to be the primary determinant for 12-positional specificity. In the 3'-untranslated region of the 12-LOX-mRNA a CU-rich, 20-fold repetitive element has been found, closely related to the differentiation control element (DICE) of the rabbit 15-LOX-mRNA which is organized by ten repeats of 19 bases. By genomic PCR the 3'-terminal part of the gene for the novel 12-lipoxygenase containing the introns 10-13 has been amplified and sequenced. The introns were very similar in length to the corresponding 15-lipoxygenase introns with 89% to 95% identical nucleotide sequences. By screening a rabbit reticulocyte library an alternative 15-lipoxygenase transcript of 3.6 kb has been detected containing a 1019 nucleotides longer 3'-untranslated region (UTR2) than the main 2.6 kb mRNA. The determination of the tissue distribution by Northern blotting showed that the 3.6 kb mRNA2 was only expressed in non-erythroid tissues, whereas the 2.6 kb mRNA1 was exclusively expressed in reticulocytes. The only cell type which has been found to express the 1.12-lipoxygenase abundantly are monocytes. The results indicate that the expression of 1.12-lipoxygenase and 15-lipoxygenase is highly regulated. The UTR2 of the 15-LOX-mRNA2 contained a novel eight-fold repetitive CU-rich motif of 23 bases length which is related but not identical to the DICE of 19 bases in the UTR1. The analysis of a genomic recombinant of the complete 9.0 kb Alox15 gene confirmed that UTR1 and UTR2 are not interrupted by an additional intron.

Angiotensin I converting enzyme and angiotensinogen gene polymorphisms related to 24-h blood pressure in paediatric type I diabetes mellitus

The aim of this study was to evaluate two putative predictive genetic markers for hypertension in children and adolescents with diabetes mellitus. Ambulatory blood pressure measurements were performed in 199 patients with type I diabetes mellitus (mean age 16.5 years, mean duration of diabetes 7.7 years) and compared to those of 1141 healthy children. The local allele frequencies were established based on a control population consisting of 181 healthy subjects. The allele frequencies of the angiotensinogen gene M235T polymorphism was nearly identical in insulin-dependent diabetes mellitus patients (MM 33%, MT 51%, TT 16%) and controls (MM 35%, MT 49%, TT 16%). In contrast, the genotype distribution of the angiotensin I converting enzyme gene insertion/deletion (I/D) polymorphism was different between patients with type I diabetes mellitus (DD 26%, ID 49%, II 25%) and the control group (DD 37%, ID 44%, II 19%) (P=0.04). Relative nocturnal systolic and diastolic pressures in patients with diabetes were higher than in healthy age- and height-matched controls; no association was found with the angiotensinogen gene M235T polymorphism. Relative nocturnal diastolic blood pressure was higher in patients homozygous for the I allele of the angiotensin I converting enzyme gene.

CONCLUSION

Nocturnal systolic and diastolic blood pressure is higher in patients with type I diabetes than in healthy children. The formerly described, but controversial, association of the M235T polymorphism with arterial hypertension could not be confirmed in this study.

Rational management of posttransplant lymphoproliferative disorder in pediatric recipients

BACKGROUND/PURPOSE

Posttransplant lymphoproliferative disorder (PTLD) is a potentially lethal complication in the pediatric transplant patient secondary to Epstein-Barr virus (EBV) infection and potent immunosuppression. PTLD may develop in up to 10% of pediatric transplant recipients with mortality rates up to 80%. The authors report their experience with the diagnosis and efficacy of aggressive sequential management of PTLD in five patients under age 5 years.

METHODS

A review of 75 pediatric liver transplant recipients on FK-506-based immunosuppression identified five biopsy-proven cases of PTLD and one probable case (8%). The probable case was a teenager, 6 months posttransplant in Spain, with mediastinal masses. No treatment or diagnosis was sought, and the patient died. The other five cases were managed with sequential therapy on an "intent-to-treat" basis with initial withdrawal of immunosuppression. If the disease progressed, patients were treated with four courses of intravenous cyclophosphamide, vincristine, Adriamycin, and intrathecal methotrexate and ara-C.

RESULTS

Five children were anti-EBV titer negative at the time of transplant. Three of five received EBV-positive donor organs and two children received EBV-negative livers. Monoclonal PTLD developed between 2 and 31 months posttransplant (mean, 15.7 months). With onset of PTLD (four B cell lymphoma, one B cell leukemia) all patients had tapering or withdrawal of immunosuppression and initiation of highdose acyclovir. Two of five patients had complete remission and resumed immunosuppression. Two patients progressed and required chemotherapy. One patient with initial response relapsed 4 months later with B cell leukemia and required chemotherapy. All five patients are alive 10 to 38 months postdiagnosis (mean, 29 months). Three patients had rejection requiring resumption of FK-506 therapy. Two patients are maintained on low-dose alternate-day prednisone and no FK-506. All patients have normal liver function. Central nervous system lymphoma developed in one child with significant neurological sequelae.

CONCLUSIONS

PTLD can develop in pediatric liver transplant recipients. Early withdrawal of immunosuppression and aggressive chemotherapy enabled us to achieve 100% patient and graft survival.

Mutations in the gene for the common gamma chain (gammac) in X-linked severe combined immunodeficiency

X-linked severe combined immunodeficiency (XSCID) constitutes a disorder of the immune system caused by mutations in the gene encoding the common gamma chain (gammac), a subunit of the IL-2, IL-4, IL-7, IL-9 and IL-15 receptors, which are necessary for lymphocyte development and function. In this study the IL2RG gene of 31 patients with severe combined immunodeficiency (SCID) was examined by nonradioactive single-strand conformation polymorphism and sequence analysis. Among the 11 patients with XSCID, ten different mutations were identified in the IL2RG gene, including eight novel mutations. Ninety percent of the mothers of the XSCID patients are carriers of the mutated allele. One patient showed low numbers of B-cells, a striking deviation from the classical B-cell-positive and T-cell-negative phenotype.

Omega-oxidation impairs oxidizability of polyenoic fatty acids by 15-lipoxygenases: consequences for substrate orientation at the active site

During oxygenation by 15-lipoxygenases, polyenoic fatty acids are bound at the active site in such a way that the omega-terminus of the fatty acids penetrates into the substrate binding pocket. In contrast, for arachidonic acid 5-lipoxygenation, an inverse head to tail orientation has been suggested. However, an inverse orientation may be hindered by the large energy barrier associated with burying the charged carboxylate group in the hydrophobic environment of the substrate binding cleft. We studied the oxygenation kinetics of omega-modified fatty acids by 15-lipoxygenases and found that omega-hydroxylation strongly impaired substrate affinity (higher Km), but only moderately altered Vmax. In contrast, omega-carboxylation completely prevented the lipoxygenase reaction; however, methylation of the additional carboxylate group restored the activity. Arg403 of the human 15-lipoxygenase has been implicated in fatty acid binding by forming a salt bridge with the carboxylate group, and thus mutation of this amino acid to an uncharged residue was supposed to favour an inverse substrate orientation. The prepared Arg403-->Leu mutant of the rabbit 15-lipoxygenase was found to be a less effective catalyst of linoleic acid oxygenation. However, the oxygenation rate of omega-hydroxyarachidonic acid was similar when the wild-type and mutant enzyme were compared, and the patterns of oxygenation products were identical for both enzyme species. These data suggest that introduction of a polar, or even charged residue, at the omega-terminus of substrate fatty acids in connection with mutation of Arg403 may not alter substrate alignment at the active site of 15-lipoxygenases.

Probing the substrate alignment at the active site of 15-lipoxygenases by targeted substrate modification and site-directed mutagenesis. Evidence for an inverse substrate orientation

For oxygenation of polyenoic fatty acids by 12- and 15-lipoxygenases the methyl terminus of the substrate constitutes the signal for the initial hydrogen abstraction. In contrast, for 5-lipoxygenases an inverse head to tail substrate orientation has been proposed. However, recent structure-based sequence alignments suggested a conserved uniform substrate orientation for 5S- and 15S-lipoxygenation. Oxygenation of 15S-HETE derivatives by various wild-type and mutant lipoxygenases was investigated, and the evidence proved an inverse substrate orientation: (i) Substrate affinity and Vmax of 15S-HETE oxygenation by arachidonic acid 15-lipoxygenases are >1 order of magnitude lower than the corresponding data for polyenoic fatty acids. 5S,15S- and 14R, 15S-DiH(P)ETE were identified as major reaction products. (ii) Methylation of the carboxylate group of 15S-HETE augmented the reaction rate and shifted the reaction specificity strongly toward 5S-lipoxygenation. In contrast, methyl arachidonate was less effectively oxygenated than the free acid. Methylation of 15S-HETrE(8,11,14), which lacks the C5-C6 double bond, was without major impact on the oxygenation rate and on the product specificity. (iii) Introduction of a bulky glycerol moiety at the carboxylic group of 15S-HETE reversed the kinetic effects of methylation and led to a 14R-oxygenation of the substrate. (iv) When the product pattern of 15S-HETE oxygenation by the recombinant wild-type rabbit 15-lipoxygenase was compared with that formed by the Arg403Leu mutant, 5S- and 8S-lipoxygenations were augmented and 14R, 15S-DiH(P)ETE formation was impaired. (v) Phe353Leu or Ile418Ala mutation of the same enzyme, which favored 12S-HETE formation from arachidonic acid, strongly augmented 8S-lipoxygenation of 15S-HETE methyl ester. These kinetic data and the alterations in the product specificity are consistent with the concept of an inverse head to tail substrate orientation during the oxygenation of 15S-HETE methyl ester and/or of free 15S-HETE by 15-LOXs. For 5S- and 8S-lipoxygenation, 15-HETE may slide into the substrate binding pocket with its carboxy terminus approaching the doubly allylic methylenes C-7 or C-10 to the non-heme iron.