Journal of Lipid Research, Vol 27, 836-857, Copyright © 1986 by Lipid Research, Inc.
Visualization by freeze fracture, in vitro and in vivo, of the products of fat digestion
MW Rigler, RE Honkanen and JS Patton
The technique of freeze fracture was used to visualize triglyceride (TG)
hydrolysis and the production of lipolytic products (LPs) in vitro and in
vivo in the presence of bile salts (BS). Three systems were investigated:
pure lipolytic products (oleic acid and monoolein) in the presence of a
pure bile salt (taurodeoxycholate (TDC)), lipolytic products produced from
TG by pancreatic lipase in the presence of a variety of bile salts, and
lipolytic products produced in the intestine of the killifish, Fundulus
heteroclitus, after fat feeding. In vitro, lamellae (4-5 nm thick with
0-8-nm water spacings) appeared on the surface of TG droplets in all
preparations with LP/BS molar ratios of 1.5 or greater and spherical
vesicles (diameter range, 20-130 nm) were produced from these lamellae.
With model killifish bile (taurocholate- cholate 1:1) at LP/BS ratios
between 1.5 and 4, homogeneous vesicles or particles (mean diameter, 23.8
nm) were produced by lipase at pH 6.9. In vivo, lamellar product phases
also occurred after fat feeding. The smallest visible LP/BS structures by
freeze fracture electron microscopy were approximately 20 nm globular
particles. Large disc- shaped micelles either were not present or were
below the resolution limit of the replica (approximately 10 nm). The
dominant aggregated lipolytic product phase was composed of multiple layers
of rough- textured lamellae. No evidence of cubic structure was seen. These
results show that lamellar and vesicular lipolytic product phases can be
intermediates in intestinal fat digestion. However, no evidence for the
direct endocytotic absorption of these product phases by the intestinal
microvillus membrane was found.
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