Editing Steroid (section)

==== Cleaved, contracted, and expanded rings ====
Secosteroids (Latin ''seco'', "to cut") are a subclass of steroidal compounds resulting, [[Biosynthesis|biosynthetically]] or conceptually, from scission (cleavage) of parent steroid rings (generally one of the four). Major secosteroid subclasses are defined by the steroid carbon atoms where this scission has taken place. For instance, the prototypical secosteroid [[cholecalciferol]], [[vitamin D3|vitamin D<sub>3</sub>]] (shown), is in the 9,10-secosteroid subclass and derives from the cleavage of carbon atoms C-9 and C-10 of the steroid B-ring; 5,6-secosteroids and 13,14-steroids are similar.<ref name="pmid20424788">{{cite journal | vauthors = Hanson JR | title = Steroids: partial synthesis in medicinal chemistry | journal = Natural Product Reports | volume = 27 | issue = 6 | pages = 887–99 | date = Jun 2010 | pmid = 20424788 | doi = 10.1039/c001262a }}</ref>

[[Norsteroid]]s ([[nor-]], L. ''norma''; "normal" in chemistry, indicating carbon removal)<ref name=iupacRF41>{{cite web | publisher = International Union of Pure and Applied Chemistry (IUPAC) | year = 1999 | title = IUPAC Recommendations: Skeletal Modification in Revised Section F: Natural Products and Related Compounds (IUPAC Recommendations 1999) | url = http://www.chem.qmul.ac.uk/iupac/sectionF/RF41.html#41 | access-date = 20 May 2014 | archive-date = 4 March 2016 | archive-url = https://web.archive.org/web/20160304041709/http://www.chem.qmul.ac.uk/iupac/sectionF/RF41.html#41 | url-status = live }}</ref> and homosteroids (homo-, Greek ''homos''; "same", indicating carbon addition) are structural subclasses of steroids formed from biosynthetic steps. The former involves enzymic [[ring expansion|ring expansion-contraction]] reactions, and the latter is accomplished ([[biomimetic synthesis|biomimetically]]) or (more frequently) through [[ring closure]]s of [[open-chain compound|acyclic]] precursors with more (or fewer) ring atoms than the parent steroid framework.<ref name=Wolfing07>{{cite journal | vauthors = Wolfing J | date = 2007 | title = Recent developments in the isolation and synthesis of D-homosteroids and related compounds | journal = Arkivoc | volume = 2007 | issue = 5 | pages = 210–230 | doi = 10.3998/ark.5550190.0008.517 | url = http://www.arkat-usa.org/get-file/19924/ | doi-access = free | hdl = 2027/spo.5550190.0008.517 | hdl-access = free | access-date = 20 May 2014 | archive-date = 1 February 2013 | archive-url = https://web.archive.org/web/20130201091834/http://www.arkat-usa.org/get-file/19924/ | url-status = live }}</ref>

Combinations of these ring alterations are known in nature. For instance, [[Sheep|ewes]] who graze on [[Veratrum|corn lily]] ingest [[cyclopamine]] (shown) and [[veratramine]], two of a sub-family of steroids where the C- and D-rings are contracted and expanded respectively via a [[biosynthesis|biosynthetic]] migration of the original C-13 atom. Ingestion of these C-nor-D-homosteroids results in birth defects in lambs: [[cyclopia]] from [[cyclopamine]] and leg deformity from veratramine.<ref name=GaoChen2012>{{cite book | veditors = Corey EJ, Li JJ | title = Total synthesis of natural products: at the frontiers of organic chemistry | vauthors = Gao G, Chen C | chapter = Nakiterpiosin | chapter-url = https://books.google.com/books?id=UT5EAAAAQBAJ | doi = 10.1007/978-3-642-34065-9 | date = 2012 | publisher = Springer | location = Berlin | isbn = 978-3-642-34064-2 | s2cid = 92690863 }}</ref> A further C-nor-D-homosteroid (nakiterpiosin) is excreted by [[Okinawa Prefecture|Okinawa]]n [[cyanobacteria|cyanobacteriosponges]]. e.g., ''[[Terpios]] hoshinota'', leading to coral mortality from black coral disease.<ref name="Uemura-2009">{{cite journal | vauthors = Uemura E, Kita M, Arimoto H, Kitamura M | date = 2009 | title = Recent aspects of chemical ecology: Natural toxins, coral communities, and symbiotic relationships | journal = Pure Appl. Chem. | volume = 81 | issue = 6 | pages = 1093–1111 | doi = 10.1351/PAC-CON-08-08-12| doi-access = free }}</ref> Nakiterpiosin-type steroids are active against the signaling pathway involving the [[smoothened]] and [[hedgehog (cell signaling)|hedgehog]] proteins, a pathway which is hyperactive in a number of cancers.{{citation needed|date=March 2019}}