feeding and Digestion 1 77 



organic esters; the acid can be a short chain like acetic or a long chain like 

 stearic, and the alcohol can be short or long chained, monohydric or poly- 

 hydric. Lipases do show some stereochemical specificity, however. '•' In mam- 

 mals waxes are not usually digested. Lipases can be tested by measuring the 

 acid produced from such substrates as olive oil, tributyrin, or ethyl butyrate. 

 Lipases are inhibited by aldehydes, cupric, mercuric, and ferric ions, and by 

 certain anions, and are actixated by some reducing agents. '■'''■' In vertebrates 

 the action of pancreatic lipase is enhanced by bile salts, sodium glycocholate 

 and sodium taurocholate. Bile salts activate lipase and also facilitate emulsifi- 

 cation of fats, thus increasing the total area of oil-water interfaces at which 

 lipases can act. If the bile duct is cut or occluded, fat digestion continues but 

 is retarded. 



In Protozoa fat digestion appears not to be very active. Fat from ingested 

 Colpidiiim is digested by Amoeba protens. ^'" In Peloinyxa ingested or 

 injected fat disappears and stained fatty acids can be seen to pass out from 

 vacuoles into the cytoplasm, where they may be synthesized into neutral fat; 

 hence lipases are present in this animal. ^^° 



Fat digestion does not occur in the gastrovascular cavity of Hydra, -- but 

 a lipase is present in this cavity in Actinia. '*^' ^- However, fat droplets may 

 be absorbed by Hydra, and lipases have been extracted from several coelenter- 

 ates.<^-"^ Rhabdocoel turbellarians digest fat rapidly. ^^^ 



The hepatopancreas of molluscs secretes or contains a lipase which is active 

 at a pH slightly above 7.0 in Mya, ^^^ Ostrea, "^" and My tikis, ''-^ and there 

 is an active lipase in Helix (pH 6.0)^'*' '*•'' and in Aplysia."^^ In Sepia both 

 the stomach juice and an extract of the liver contain a lipase with a pH 

 optimum at 6.0, whereas the pancreatic lipase works best at 6.35. ^^^ In 

 annelids the lipases are less important than they are in molluscs. A lipase (pH 

 optimum 8.2-8.4) is present in the leech Haemopsis, but not in Hiriido. ■* 



Arthropods make good use of fats. In the crayfish a lipase is active on 

 methylbutyrate and olive oil over a broad pH optimum of 5.5 to 7.0, and the 

 stomach juice contains a fat-dispersing agent. "^"' ^"^ Extracts of Daphnia 

 have a weak lipase. "^'^ In Liviulus the stomach lipase is more active than that 

 from the hepatopancreas (pH optimum 7.7). ^^■' The lipases of the crayfish 

 and of several molluscs (Aplysia, Helix, Octopus) resemble liver esterases of 

 vertebrates in acting better on lower esters, such as tributyrin and methyl- 

 butyrate, than on fats such as olive oil. ^^^^ ^'" 



Several insects have been shown to have lipases, particularly in the mid- 

 gut. ^"" The mid-gut lipase can be regurgitated into the crop of the cock- 

 roach. ^ The digestive tract of the silkworm, however, contains no lipase, or 

 if one is present it is very weak. ^■^"* The wax moth larva Galleria uses beeswax 

 as its principal food, nearly half of the wax consumed being digested and 

 assimilated. '^^ The mid-gut contains a lipase, ^^•' but larval extracts are 

 ineffective on myricin, the ester of higher alcohols which composes much of 

 beeswax. Bacteria in the digestive tract probably change the wax so that it 

 can be digested. ^-' '^^ 



The hepatic glands of the prochordate Glossobalaniis contain a lipase active 

 on triacetin at pH 6-7. ^'^ 



Digestion of fat in vertebrates is primarily intestinal. A gastric lipase has 

 been reported with pH optima of 6.3 in dog and rabbit, 5.5 in man, 8.6 in 



