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Phosphatidylcholine as substrate for human pancreatic phospholipase A 2 . Importance of the physical state of the substrate
Author(s) -
Borgström Bengt
Publication year - 1993
Publication title -
lipids
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.601
H-Index - 120
eISSN - 1558-9307
pISSN - 0024-4201
DOI - 10.1007/bf02535932
Subject(s) - phosphatidylcholine , vesicle , chemistry , substrate (aquarium) , micelle , chromatography , liposome , phospholipid , aqueous solution , biochemistry , organic chemistry , membrane , oceanography , geology
Abstract The long‐chain phosphatidylcholine/sodium cholate aqueous system as substrate for human pancreatic phospholipase A 2 (PLA 2 ) was investigated. At a constant phosphatidylcholine (PC) concentration of 8 mM, the enzyme activity increased with a decrease in cholate (C) concentration up to a PC/C ratio of approximately 0.8 and then rather abruptly decreased to lower values at a ratio above 1.5. At ratios between 0.8 and 1.5, an increasing lag phase in the PLA 2 activity was seen, indicating a progressive decrease in substrate availability to the enzyme. Reaction mixtures with a PC/C ratio of up to 0.67 were optically clear solutions composed of mixed bile salt/PC micelles of increasing mixed micellar aggregate size. Ratios between 0.67 and 1.5 were characterized by an increase in turbidity (at 330 and 450 nm) due to increasing formation of vesicles or liposomes. Above a PC/C ratio of 1.5, a sharp increase in turbidity was seen due to increasing formation of bilayer structures other than vesicles. Pure vesicles obtained by dialysis of mixed micellar solutions were not hydrolyzed by the enzyme. Addition of bile salts reversed the inhibition which was accompanied by a decrease in turbidity. Phosphatidylcholine was preferred as substrate for human PLA 2 when present in large mixed disc‐like bile salt micelles. Vesicular or other types of lamellar liquid‐crystalline phases of long‐chain phosphatidylcholine did not serve as substrate for PLA 2 .