Abstract— The calcium‐dependent incorporation of l‐[3‐3H]serine and [1,2‐14C]ethanol‐amine into the phospholipid of isolated subcellular fractions from chick brain was studied and the properties of incorporation were examined. The microsomal fraction was found to possess the highest rate of incorporation and was able to convert under optimal conditions about 120 nmol of labelled serine and 220 nmol of ethanolamine/g of fresh brain microsomes/h. The requirement for Ca2+ ion appeared to be absolute. Mg2+ ion caused a gradual reduction in the existing enzymic activity, only when pre‐incubated with microsomes and labelled bases before adding Ca2+ ion. The incorporation of serine and ethanolamine was actively inhibited by Hg2+, Co2+, Cu2+ and Mn2+ ions, and was abolished by ethylenediamine tetra‐acetate treatment. Ethanolamine, but not choline, inositol or carnitine, competitively inhibited serine incorporation, while d‐serine had slight effect. Conversely, l‐serine inhibited competitively the incorporation of ethanolamine.
The greater part of the incorporated serine (85 per cent) was localized in microsomal phosphatidylserine, while a small percentage was found in phosphatidylethanolamine. Similarly, 90 per cent of the incorporated ethanolamine was confined to phosphatidylethanolamine and a small percentage was found in the plasmalogen derivative. The mechanism of serine and ethanolamine incorporation was investigated. The results are compared with those published for similar mammalian and non‐mammalian systems.
Cholesterol biosynthesis has been examined using rat liver slices in vitro from 2‐14C‐acetate and 2‐14C‐mevalonate, in the presence of β‐benzal butyric acid (BBA) and its metabolite, α‐hydroxy β‐benzal butyric acid (HBBA), both of which are postulated to act as potential hypocholesterolemic agents. Procedures have been devised to follow radioactivity incorporation of these precursors into the squalene, lanosterol and cholesterol fractions. The results show that cholesterol synthesis from labeled acetate is noticeably inhibited by BBA final concentrations as small as 10 μM, while the rate of labeling is much less inhibited by HBBA. When acetate is replaced by labeled mevalonate, cholesterol synthesis is hardly inhibited by both BBA and HBBA. The results indicate that BBA probably affects some of the reactions which lead acetate to mevalonate formation. Acetyl‐CoA: ligase (E.C.6.2.1.1) and acetyl‐CoA acetyl transferase (E.C.2.3.1.) therefore have been examined. Ligase activity is substantially inhibited only by 1 mM concentration of BBA and HBBA, whereas the transferase enzyme is unaffected. BBA probably affects other reactions in the metabolic sequence which converts acetate into mevalonate.
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