Melzer's reagent
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190564127
2008-02-11T06:14:57Z
Peter G Werner
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/* History */
'''Melzer's reagent''' (also known as '''Melzer's solution''' or informally as '''Melzer's''') is a [[chemical reagent]] used by [[mycologist]]s to assist with the identification of [[fungi]].
Melzer's reagent is an [[aqueous solution]] of [[chloral hydrate]], [[potassium iodide]], and [[iodine]]. Depending on the formulation, it consists of approximately 2.50-3.75% potassium iodide and 0.75–1.25% iodine, with the remainer of the solution being 50% water and 50% chloral hydrate.<ref name=largent>Largent D, et al. 1977. ''How to Identify Mushrooms to Genus III: Microscopic Features''. Arcata, CA: Mad River Press. ISBN 0-916-422-09-7. p 25–26.</ref><ref>Miller OK, Miller H. 2006. ''North American Mushrooms: A Field Guide to Edible and Inedible Fungi''. Guilford, CT: Falcon Guide. ISBN 0762731095. p 549.</ref>
Melzer's is used by exposing fungal tissue or cells to the reagent, typically in a [[microscope slide]] preparation, and looking for any of three color reactions:
* [[Amyloid (mycology)|Amyloid]] or Melzer's-positive reaction, in which the material reacts blue to black.
* Pseudoamyloid or [[dextrinoid]] reaction, in which the material reacts brown to reddish-brown.
* Inamyloid or Melzer's-negative, in which the tissues do not change color, or react faintly yellow-brown.<ref name=largent/>
Among the amyloid rection, two types can be distinguished:
* Euamyloid reaction, in which the material turns blue without [[potassium hydroxide]] (KOH)-pretreatment.
* Hemiamyloid reaction, in which the material turns red in [[Lugol's solution]], but shows no reaction in Melzer's reagent; when KOH-pretreated it turns blue in both reagents.<ref name=baral>Baral H-O. 1987. [http://www.cybertruffle.org.uk/cyberliber/59575/0029/0399.htm Lugol's solution/IKI versus Melzer's reagent: Hemiamyloidity, a universal feature of the ascus wall.] ''Mycotaxon'' 29:399–450.</ref>
Melzer's reactions are typically almost immediate, though in some cases the reaction may take up to 20 minutes to develop.<ref name=largent/>
The function of the chemicals that make up Melzer's reagent are several. The chloral hydrate is a [[clearing agent]], bleaching and improving the transparency of various dark-colored microscopic materials. The potassium iodide is used to improve the [[solubility]] of the iodine, which is otherwise only semi-soluble in water. Iodine is thought to be the main active staining agent in Melzer's; it is thought to [[iodine test|react with starch-like polysaccharides]] in the cell walls of amyloid material, however, its mechanism of action is not entirely understood. It has been observed that hemiamyloid material reacts differently when exposed to Melzer's then it does when exposed to other IKI solutions such as Lugol's, and that in some cases an amyloid reaction is shown in material that had prior exposure to KOH, but an inamyloid reaction without such pretreatment.<ref name=baral/><ref>Kohn LM, Korf RP. 1975. [http://www.cybertruffle.org.uk/cyberliber/59575/0003/001/0165.htm Variation in ascomycete iodine reactions: KOH pretreatment explored.] ''Mycotaxon'' 3:165–172.</ref>
Melzer's degrades into a cloudy precipitate when combined with alkaline solutions,<ref name=largent/> hence it cannot be used in combination or in direct series with such common mycological reagents such as potassium hydroxide or [[ammonium hydroxide]] solutions. When potassium hydroxide is used as a pretreatment, the alkalinity must be first neutralized before adding Melzer's.
The reagent is toxic to humans due to the presence of iodine and chloral hydrate.
==History==
Melzer's reagent is part of a class of iodine/potassium iodide (IKI)-containing reagents used in biology; [[Lugol's iodine]] is another such formula. The reagent was first described in 1924 and takes its name from its inventor, the mycologist [[Václav Melzer]], who modified an older chloral hydrate-containing IKI solution developed by botanist [[Arthur Meyer (botanist)|Arthur Meyer]].<ref name=baral/> Melzer was a specialist in ''[[Russula]]'', a genus in which the amyloidy on the [[spore]] ornamentation or entire spore is of great taxonomic significance.<ref>The Russulales News Team. 2007. Introduction to the Russulales: [http://www.mtsn.tn.it/russulales-news/in_characteristics.asp Characteristics of the russuloid fungi], ''Russulales News'' (website).</ref>
==References==
<references/>
==Further reading==
* Blackwell M, et al. 2001. The presence of glycine betaine and the dextinoid reaction in basidiomata. ''Harvard Papers in Botany'' 6:35–41.
* Leonard LM. 2006. Melzer's, Lugol's, or iodine for identification of white-spored Agaricales? ''McIlvainea'' 16(1).
* Rossman AY. 1980. The iodine reaction: Melzer's vs. IKI. ''MSA newsletter'' 31:22.
[[Category:Mycology]]
[[Category:Chemical tests]]
[[de:Melzers Reagenz]]