Biosynthesis of Cyclopamine from Cholesterol

Cyclopamine is isolated from the plant Veratrum californicum in the late 1960s, commonly known as corn lily. Cyclopamine is a naturally alkaloid that has inhibition activity on HH pathway by blocking the activation of Smo. This article will introduce its discovery and biosynthesis method.

Discovery of Cyclopamine

Veratrum californicum, also known as the California false hellebore, is a lily of the family of melanthiaceae that is native to the northern hemisphere but is especially abundant in the mountainous area of the western USA. In the middle of the 20th century, a random batch of lambs in sheep herds in the state of Idaho was born with severe craniofacial defects. Up to 25 % of newborn lambs of sheep grazing in the mountains of central Idaho were affected. 

The affected sheep herders asked the Department of Agriculture for help when the disease had reached endemic proportions and the economic loss was no longer tolerable. Lynn F. James, one of the scientists who was sent, lived with the sheep for three summers in the late 1950s and discovered that in times of drought the sheep moved to higher grounds and grazed on the abundantly growing flower veratrum californicum. Richard F. Keeler of the Poisonous Plant Research Laboratory later figured out the connection between the consumption of veratrum californicum by pregnant sheep and the occurrence of cyclopia in their offspring.

In the basis of these initial studies, an extraction method was developed that allowed the alkaloids of veratrum californicum to be isolated for investigation of their ability to induce cyclopia in embryos of pregnant sheep.

It was shown that only three compounds possessed this ability, and in various potencies, namely the known alkaloid jervine, alkaloid X (later identified as 3-glucosylcyclopamine and termed cycloposine), and a third more active steroidal alkaloid resembling jervine that induced cyclopia in the offspring of 12 of 40 pregnant sheep. A definitive structural characterization of the new compound as 11-deoxojervine was gained from the degradation of jervine by a Wolff–Kishner reduction. The product of this transformation was identical in every respect to the newly isolated steroidal alkaloid. In accordance with the epic by Homer about the odyssey in which the encounter of Odysseus with the cyclops Polyphemus is described, Keeler named this new natural product cyclopamine.

Biosynthesis Method

Figure 1. Biosynthesis of cyclopamine from cholesterol in veratrum grandiflorum.[1]

Cholesterol is transformed in the plant through a number of reactions (some have only been assumed) into the parent compound of this class of natural products: solanidine. In a series of oxidative and reductive processes, first the piperidine is synthesized and then the E ring of solanidine is formed in a reaction driven by adenosine triphosphate (ATP). A stereounspecific oxidase subsequently hydroxylates at the C12-position to form the 12a-hydroxy product (rubijervine), which is enriched in the rhizomes, and the 12b-hydroxy product (epirubijervine). Only the hydroxy group of the latter compound is converted into a phosphate leaving group and rearranges through a Wagner–Meerwein type reaction into the C-nor-D-homosteroid skeleton. Reductive opening of the pyrollidine ring (E ring) gives husokinidine, which is finally oxidatively cyclized to form the tetrahydrofuran ring of cyclopamine.

Reference

[1] Philipp Heretsch; Lito Tzagkaroulaki; Athanassios Giannis. Cyclopamine and Hedgehog Signaling: Chemistry, Biology, Medical Perspectives[J]. Angewandte Chemie International Edition, 2010, 49 20: 3418-3427.