Ian J. Cartwright

Direct Evidence for a Two-step Assembly of ApoB48-containing Lipoproteins in the Lumen of the Smooth Endoplasmic Reticulum of Rabbit Enterocytes

The aim of this study was to investigate the types and characteristics of chylomicron precursors in the lumen of the secretory compartment of rabbit enterocytes. Luminal contents were separated into density subfractions in two continuous self-generating gradients of different density profiles. In enterocytes from rabbits fed a low fat diet, newly synthesized and immunodetectable apoB48 was only in the subfraction of density similar to high density lipoprotein (dense particles); the luminal triacylglycerol (TAG) content was low and only in the subfraction of density similar to that of chylomicrons/very low density lipoproteins (light particles). After feeding fat, newly synthesized, and immunodetectable apoB48 was in both dense (phospholipid-rich) and light (TAG-rich) particles. Luminal TAG mass and synthesis increased after fat feeding and was only in light particles. Pulse-chase experiments showed that the luminal-radiolabeled apoB48 lost from the dense particles was recovered in the light particles and the secreted chylomicrons. All of the light particle lipids (mass and newly synthesized) co-immunoprecipitated with apoB48. However, in the dense particles, there was a preferential co-precipitation of the preexisting rather than newly synthesized phospholipid. Assembly of apoB48-containing TAG-enriched lipoproteins is therefore a two-step process. The first step produces dense apoB48 phospholipid-rich particles, which accumulate in the smooth endoplasmic reticulum lumen. In the second step, these dense particles rapidly acquire the bulk of the TAG and additional phospholipid in a single and rapid step.

Lipid Abnormalities in Serum in Batten's Disease

two patient registers and conducted computer searches at North Dakota’s comprehensive diagnostic center and the state’s institution for developmentally disabled persons in an attempt to identify patients with co-existing tuberous sclerosis and PDD. We identified three adults and one child with tuberous sclerosis who met criteria for PDD (Table I). From our PDD registers and the patient register of the state’s clinical genetics program we identifed 12 patients in the state with tuberous sclerosis; a prevalence of I 84 per 100,000. Our estimate of the prevalence of PDD was 3.3 per 10,000. The estimated prevalence of co-occurrence of these two conditions (given random assortment and the above prevalence estimates) would be 6.07 x lo’, or approximately six cases of co-existing tuberous sclerosis and PDD per billion. Thus it would be extremely unlikely that we could identify four patients with tuberous sclerosis and PDD in a state with 652,695 residents unless there is a link between the two disorders. For all three of our adult patients, by history, the onset of autism was before two years of age, and all three continue to meet criteria for autism. These findings conflict with Creak’s contention that symptoms common to the two disorders occur for only a short period. It remains unclear why the genetic literature has not generally reported PDD as a clinical manifestation of tuberous sclerosis. WAYNE FISHER, PH.D.* JACOB KERBESHIAN, M.D.? LARRY BURD, M.S.

Post-translational events in the intracellular transit of apolipoprotein-B: modulation by dietary lipids

PAUL KOLSTOE, M.A.* *Grafton State School, Grafton, ND 58237 t University of North Dakota School of Medicine, Grand Forks’ ND 58202

Intracellular events in the assembly of chylomicrons in rabbit enterocytes

Medical Center, Rehabilitation Hospital, Grand Forks, ND 58202

Transit and sorting of apolipoprotein B within the endoplasmic reticulum and Golgi compartments of isolated hepatocytes from normal and orotic acid-fed rats.

Triacylglycerol and cholesteryl esters are transported in the blood as components of the plasma lipoproteins. Endogenous triacylglycerol synthesized by the liver is secreted into the plasma in the form of VLDL, which consists of a droplet of non-polar lipid (predominantly triacylglycerol with a variable amount of cholesteryl ester), stabilized by an outer shell of phospholipid, cholesterol and protein. Apolipoprotein-B (apo-B) is the major protein of VLDL. Although apo-B is an essential component of VLDL, secretion is driven by provision of substrates to the liver for triacylglycerol synthesis. These include glucose and nonesterified fatty acids. The composition and size of VLDL is influenced by the substrate provided (Sniderman & Cianflone, 1993). Dietary experiments and experiments using isolated hepatocytes have shown that provision of carbohydrates increases synthesis of triacylglycerol-enriched, larger, lighter VLDL (VLDLl), while provision of fatty acids results in production of smaller, denser VLDL (VLDL2). Since VLDLl are cleared rapidly from the circulation, while VLDL2 are cleared more slowly and are converted to the atherogenic LDL, the nature of VLDL secreted by the liver is an important determinant of cardiovascular risk (Packard et al. 1984; Shepherd & Packard, 1987). An understanding of the mechanisms involved in regulation of the assembly of VLDL and the factors determining their composition is, therefore, extremely important.