[Histonet] cyclic dextrins?

[John Kiernan]

What are the "cyclic dextrins" that you are asked to stain in "formalin fixed tissue"? Are they cyclodextrins, dextrins or dextrans?  
 
If the fixative was mostly water, dextrins (and dextrans if present) have probably all been dissolved out. These oligo- and polysaccharides are soluble in water. Unlike proteins, they do not react with formaldehyde to form insoluble products. Read on.

Cyclodextrins are made by the action of a bacterial amylase on starch. The long chain of 1-->4-linked  glucose units is broken down to shorter chains (6, 7 or 8 units), with the ends of each chain joined 1-->4 to make a ring. There are three types of cyclodextrin: alpha (smallest, with 6 glucose units), beta (7), and gamma (8 units). In three dimensions the ring is a shell or cage, with hydrophilic -OH groups of glucose units on the outside and a hydrophobic interior. A hydrophobic organic molecule can be enclosed within a cyclodextrin molecule without chemical change. The complex, known as a clathrate, is soluble in water; this provides a way to make an aqueous solution of an otherwise insoluble hydrophobic substance. 
 
The ordinary non-cyclic dextrins (3 types) are produced by either dry heating (toast) or acid hydrolysis of starch. Properties vary with method of production. They are soluble in water and precipitated by addition of alcohol. The information in this and the previous paragraph is from the Merck Index (which gives some references) and from an old biochemistry textbook for medical students (West & Todd, 1956). 
 
I do not have any authoritative statement about the solubilities of the three cyclodextrins in liquids other than water. They may, like simple dextrins, be insoluble in alcohols. You can be sure they won't dissolve in xylene or paraffin. 
 
Dextrins must not be confused with dextrans, which are bacterial polysaccharides composed of glucose units linked 1-->6 (in contrast to the 1-->4 of plants and animals). Dextrans are the protein substitutes in traditional artificial plasma expanders (Rheomacrodex when I was young; is it still used?) and they can end up in the renal tubules. According to Lillie's big book, which cites a paper by Bob Mowry (Am. J. Path. 29:523, 1953), water must be avoided at all stages prior to staining dextran. That means a non-aqueous fixative and floating out paraffin sections on 95% alcohol, not water. The reagents for staining (periodic acid-Schiff) should be made up in alcohol rather than water. 
 
Mowry's and Lillie's simple rules probably apply also to the histochemical detection of cyclo- and other dextrins: Non-aqueous fixative --> wax --> flatten on warm alcohol  --> dewax with xylene and then -->  alcoholic PAS method. Does this help?  Please reply to all.

John Kiernan
Anatomy, UWO
London, Canada
---
----- Original Message -----
From: Cynthia Swanson <CSwanson <@t> biotechnics-inc.com>
Date: Monday, October 22, 2007 13:13
Subject: [Histonet] cyclic dextrins?
To: histonet <@t> lists.utsouthwestern.edu

> I need a protocol to detect cyclic dextrins in formalin fixed tissue
> (dextrin is an acidic hydrolysis digestion product of starch). I 
> didn'tfind an appropriate protocol in the archives, any help 
> would be greatly
> appreciated!
> 
>  
> 
> Cyndi
> 
>  
> 
>  
> 
> Cynthia Swanson
> 
> Biotechnics
> 
> 310 Millstone Dr.,
> 
> Hillsborough, NC 27278
> 
> (919) 245-3114 x105
> 
> cswanson <@t> biotechnics-inc.com
> 
>  
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