Biosynthesis of p-Sitosterol and Pentacyclic Triterpenes of Salvia oficinaZk*

As Heftmann, Bennett, and Bonner have indicated (l), considerable information has accumulated concerning the biosynthesis of sterols in animals (2) and fungi (3), yet practically nothing is known about the biogenesis of typically higher plant sterols or the numerous pentacyclic triterpenes that are widely distributed throughout the plant kingdom. As yet, it cannot be assumed that the over-all biosynthetic mechanism for sterols and cyclic triterpenes is the same in all tissues. One particular step, however, may be common to the formation of both types of polycyclic compounds, namely, that involving the cyclization of squalene. Since squalene seems to be a “universal precursor” of fungi and animal sterols (4), this acyclic triterpene might be expected to play a similar role in the formation of higher plant sterols, of which &sitosterol is a representative example. Although Arigoni (5) has presented preliminary experimental evidence that squalene cyclizes to @sitosterol and to certain pentacyclic triterpenes in higher plants, more experimental data seem warranted. In studying the biosynthesis of sterols and pentacyclic triterpenes, there are some obvious advantages in selecting a plant that contains both of these substances. Since fl-sitosterol and several common members of the acidic pentacyclic triterpene group are found together in the Labiatae family (B), the latter has seemed a suitable source for investigation. In beginning this study with Xaluia oficinalis, which contains p-sitosterol (7), oleanolic acid, ursolic acid (Fig. l), and ursolic acid II (8): it was recognized that the triterpene acids represent higher oxidation states of a previously formed /%amyrin or amyrin nucleus. How this influences the biosynthesis or over-all metabolism of these substances was not predictable from existing data. Therefore, the present work was still largely exploratory. The ultimate objective of the work described in this and the third paper of this series (9) was to establish a direct relationship between squalene, p-sitosterol, and pentacyclic triterpene. A preliminary investigation (10) established that isolated organs of S. oqicinalis incorporated sodium acetate-2-C4 into sterol and acidic triterpenes. The appropriate radioactive fractions were extensively diluted with nonradioactive sterol and triterpene. Additional groundwork seemed necessary. In the present investigation, particular attention was given to purifying the sterol and triterpene fractions without diluting them. Some immediate objectives of the present work were the following: (a) determination of the contribution of sodium acetate-2-Cl4 to sterol and triterpene in the various plant organs; (b) a prelimi-