The H+‐dependent uptake system responsible for the enteral absorption of oligopeptides and orally active β‐lactam antibiotics was functionally reconstituted into liposomes. Membrane proteins from rabbit small intestinal brush border membrane vesicles were solubilized with n‐octyl glucoside and incorporated into liposomes using a gel filtration method. At protein/lipid ratios of 1:10 and 1:40, the uptake of the orally active α‐amino‐cephalosporin, D‐cephalexin into proteoliposomes was stimulated by an inwardly directed H+ gradient and was protein‐dependent. In these proteoliposomes the binding protein for oligopeptides and β‐lactam antibiotics of Mr 127000 could be labeled by direct photoaffinity labeling with [3H]benzylpenicillin revealing an identical binding specificity as in the original brush border membrane vesicles. The uptake system for β‐lactam antibiotics and oligopeptides showed a remarkable stereospecificity; only D‐cephalexin was taken up by intact brush border membrane vesicles, whereas the L‐enantiomer was not taken up to a significant extent. This stereospecificity for uptake was also seen after reconstitution of solubilized brush border membrane proteins into liposomes demonstrating a functional reconstitution of the peptide transporter. Both enantiomers however, bound to the 127‐kDa binding protein as was shown by a decrease in the extent of photoaffinity labeling of the 127‐kDa protein in the presence of both enantiomers. After reconstitution of subfractions of brush border membrane proteins obtained by wheat germ lectin affinity chromatography into proteoliposomes, only liposomes containing the 127‐kDa binding protein showed a significant uptake of D‐cephalexin whereas the L‐enantiomer was not transported. The uptake rates for D‐cephalexin into proteoliposomes correlated with the content of 127‐kDa binding protein in these liposomes as was determined by specific photoaffinity labeling with [3H]benzylpenicillin. The purified 127‐kDa binding protein was also reconstituted into liposomes and its ability for specific binding of substrates as well as stereospecific uptake of cephalexin could be restored. These results indicate that the binding protein for oligopeptides and β‐lactam antibiotics of Mr 127000 mediates the stereospecific and H+‐dependent transport of orally active β‐lactam antibiotics across the enterocyte brush border membrane. We therefore suggest that this 127‐kDa binding protein is the intestinal peptide transport system (or a component thereof).

No abstract

Eine Totalsynthese der enantiomerenreinen 19-Nor-Steriode Ostron (la), 19-Norandrost-4-en-3.17-dion (Za), Ostradiol-17b (3a) und 19-Nortestosteron (4a) wird beschrieben. Sie folgt dem A + D +AD +ABCD-Aufbauprinzip, durchlauft ein lichtinduziertes, kinetisch instabiles o-Chinodimethan-Derivat und verwendet den chiralen Synthesebaustein (R)-2-Vinyl-l,1 -cyclopropandicarbonsaure-dimethylester (12b). 12b ist durch eine Asymmetrie induzierende ScW-Reaktion (s. Abb. 2) bequem zuganglich. Bei der Synthese fallt 12b enantiomerenrein an. Das analytische Procedere offenbart, daD 12b mit einer optischen Reinheit oder mit einem EnantiomerenUberschuR jeweils von 95% tustande kommt. Asymmetric Total Synthesis of 19-Nor-Steroids V~Q a Photochemical Key Reaction: Enanliomerically Pure Target Compounds 1.3)A total synthesis of the enantiomerically pure 19-nor-steroids estrone (la), 19-norandrost-4-en-3.17-dione (2a), estradiol-170 (3a), and 19-nortestosterone (4a) is described. It follows the A + D -AD-ABCD sequence, proceeds via a kinetically unstable o-quinodimethane derivative obtained in a light induced reaction, and employs the chiral synthetic building block dimethyl ( R ) -2-vinyl-l , 1-cyclopropanedicarboxylate (12b). The latter is easily accessible by an asymmetry inducing SCNr reaction. During the synthetic procedure it is isolated enantiomerically pure. The analytic procedure reveals that 12b is formed with an optical purity or an enantiomeric excess, respectively, of 95%. Nach den racemischen nun die enantiomerenreinen 19-Nor-SteroideEine Totalsynthese der racemischen 19-Nor-Steroide rac-la bis rac-4a (s. Schema 1) wurde kiirzlich *) ausfiihrlich beschrieben. Sie ging von wohlfeilen Industriechemikalien aus, enthielt eine photochemische Schlusselreaktion und folgte dem A + D +AD -ABCD-Aufbauprinzip: Zwei Synthesewurzeln miindeten in einen Synthesestamm, der sich zu den diversen Zielverbindungen vermeigte (s. Abb. 1).Kernstuck der fruheren Mitteilung waren die beiden Transformationen, bei denen die diseco-steroidale Schlusselverbindung rac-9a zum entsprechenden Photo-Enol rac-10 und dieses zu den steroidalen Cycloaddukten rac-7b und rac-8b isomerisierten (s. Schema 2).

2,4‐Cyclohexadien‐1‐one des Formeltyps 1 bis 5 (s. Abb. 1) liefern Basiswerte und Positionsinkremente für UV‐ und 13C‐NMR‐Spektren. Diese Kenndaten gestatten es, zwischen Konstitutionsisomeren zu unterscheiden, die ein und derselben Spalte des Graphen von Abb. 1 angehören. Damit ist es z. B. möglich geworden, die Konstitutionsisomeren 6 und 8 bzw. 12 und 14 sicher zu identifizieren.

The complete manuscript of this communication appears in: Angew. Chem. Suppl. 1982, 1777. DOI: