Vol. 282, Issue 2, 648-656, 1997

Endogenous Natriuretic Factors 6: The Stereochemistry of a Natriuretic -Tocopherol Metabolite LLU-¹

Darko Kantoci, William J. Wechter, E. David Murray, Jr., Sally A. Dewind, Dan Borchardt and Saeed I. Khan

Laboratory of Chemical Endocrinology, Loma Linda University School of Medicine, Loma Linda, California (D.K., W.J.W., E.D.M. Jr., S.A.D.), Analytical Chemistry Instrumentation Facility, University of California, Riverside, California (D.B.) and Department of Chemistry and Biochemistry, University of California, Los Angeles, California (S.I.K)

2,7,8-Trimethyl-(S)-2-(-carboxyethyl)-6-hydroxy chroman (S-LLU-) isolated from human uremic urine is apparently an oxidative side-chain degradation product of -tocopherol. This compound exhibits natriuretic activity in vivo and it appears to mediate the inhibition of the 70 pS K⁺ channel in the apical membrane of the thick ascending limb of the nephron. The stereochemistry at the C-2 of LLU- has been unequivocally established to be S(+) by X-ray crystallographic analysis of a diastereomeric amide derivative. It was also established that the chroman ring oxidation of S-LLU- proceeded without racemization at C-2. This finding can be extended to nonepimerization at C-2 of - tocopherols (Vitamin E) during side-chain oxidation and stereospecificity (retention or inversion) of oxidative opening of the chroman ring. The resolution of the enantiomers of the parent compound and derivatives was accomplished by chiral high-performance liquid chromatography. The stereospecific enzymatic hydrolysis by an array of commercially available enzymes of the racemic methyl ester of LLU- was investigated. The lipase from Humicola languinosa appears to be the best enzyme for resolution by selective hydrolysis of the racemic methyl ester.