Tuesday, January 22, 2008

More Bout Leeches

arting from the anterior sucker is the jaw, the Pharynx which extends to the crop, which leads to the Intestinum, where it ends at the posterior sucker. The crop is a type of stomach that works like an expandable storage compartment. The crop allows a leech to store blood up to five times its body size; because of this ability to hold blood without the blood decaying, due to bacteria living inside the crop, medicinal leeches only need to feed two times a year.

It was long thought that bacteria in the gut carried on digestion for the leech instead of endogenous enzymes which are very low or absent in the intestine. Relatively recently it has been discovered that all leeches and leech species studied do produce endogenous intestinal exopeptidases which can unlink free terminal-end amino acids, one amino acid monomer at a time, from a gradually unwinding and degrading protein polymer. However, unzipping of the protein can start from either the amino (tail) or carboxyl (head) terminal-end of the protein molecule. It just so happens that the leech exopeptidase (arylamidases), possibly aided by proteases from endosymbiotic bacteria in the intestine, starts from the tail or amino protein, free-end, slowly but progressively removing many hundreds of individual terminal amino acids for resynthesis into proteins that constitute the leech. Since leeches lack endopeptidases, the mechanism of protein digestion can not follow the same sequence as it would in all other animals where exopeptidases act sequentially on peptides produced by the action of endopeptidases. Exopeptidases are especially prominent in the common North American worm-leech Erpobdella punctata. This evolutionary choice of exopeptic digestion in Hirudinea distinguishes these carnivorous clitellates from Oligochaeta.

Deficiency of digestive enzymes (except exopeptidases) but more importantly deficiency of vitamins, B complex for example, in leeches is compensated for by enzymes and vitamins produced by endosymbiotic microflora. In Hrudo medicinalis these supplementary factors are produced by an obligatory symbiotic relationship with a single bacterium species, Aeromonas hydrophila, which maintains itself in pure culture by secreting an antibiotic known to medicine since the 19th century, well before Fleming's 1929 discovery of penicillin. Non-bloodsucking leeches such as E. punctata are host to three bacterial symbionts, Pseudomonas sp., Aeromonas sp., and Klebsiella sp. (a slime producer). The bacteria are passed from parent to offspring in the cocoon as it is formed.