A better mode for counting ceramides
Ceramides are important lipids in the intercellular spaces of the stratum corneum or SC, the outer layer of the epidermis. These structurally diverse and complex sphingolipids contain derivatives of sphingosine bases in amide linkage with a variety of fatty acids. Epidermal ceramides have varying chain length, type and degree of hydroxylation, saturation and other variables. They play an essential role in structuring and maintaining the water permeability of the skin.
Henry Vandyke Carter/Wikimedia Commons
Researchers have found a new way to count ceramides in the stratum corneum,
the outermost layer of the skin, shown at the top of this illustration of all layers
of the epidermis.
Although researchers have found ceramides with long-chain bases, or LCBs, of varying lengths in the stratum corneum, no one has yet published a quantitative analysis. Previous measurements using liquid chromatography-mass spectrometry or liquid chromatography-tandem mass spectrometry, known as LC-MS and LC-MS/MS, have reported between 100 and 400 ceramide species. However, these studies have drawbacks; ceramide species are reported solely based on the sum of their LCB and FA chain lengths, or the coverage of ceramide species is less thorough due to the low sensitivity of the production ion scan analysis.
Madoka Suzuki and a team of researchers at Hokkaido University, Japan, recently performed LC-MS/MS on SC ceramides using the specialized multiple reaction monitoring mode, which can detect and quantify multiple molecular species in a single measurement. Using this method, the researchers discovered individual ceramide species that differed in both LCB and FA chain length and quantified the largest number of ceramide species reported to date (1,327 unbound ceramides and 254 protein-bound ceramides). Their recent study in the Journal of Lipid Research provides a molecular basis for elucidating human SC-ceramide diversity and the pathogenesis of skin disorders.
A new way to measure lipoprotein(a).
Lipoprotein(a), or Lp(a), is a genetic risk factor for cardiovascular disease and aortic stenosis. Similar in composition to low-density lipoprotein, Lp(a) is characterized by the carbohydrate-rich apolipoprotein(a) or apo(a), which confers distinct pathophysiological and metabolic characteristics.
Apo(a) contains repeated kringle structures, or KIV, which are protein domains that fold into large loops that are stabilized by three disulfide bonds. KIV is formed by 10 subtypes (KIV1 to KIV10), each present as a single copy except KIV2, which is present in a variable number, ranging from one to more than 40 identical repeats, in different individuals. Thus, humans can have apo(a) molecular weight ranging from about 300 to 800 kilodaltons. This means that plasma levels of Lp(a) are over- or underestimated, making it challenging to accurately measure Lp(a) levels.
Santica Marcovina of Medpace Reference Laboratories and a team of researchers recently developed a new isoform-independent sandwich Lp(a) enzyme-linked immunosorbent assay, or ELISA, to measure Lp(a). The test captures Lp(a) with monoclonal antibody LPA4 directed primarily against an epitope in apo(a) KIV2 and detects it with monoclonal antibody LPA-KIV9 directed against a single antigenic site present on KIV9.
When tested on 64 samples with known apo(a) isoforms, the new test performed as well as standard methods. A recent article in the Journal of Lipid Research describes the development and validation of the assay, which the researchers believe will benefit research laboratories trying to eliminate the confusing bias generated by heterogeneous apo(a) isoform sizes.
Uncovering a source of metabolized cholesterol
Once absorbed by the body, cholesterol is converted to cholesteryl esters to facilitate efficient transport. Although lipoproteins can absorb free cholesterol, it is limited to their outer surface. Conversion to cholesteryl esters allows more cholesterol to be packaged into lipoproteins, greatly increasing the capacity of the lipoproteins and allowing cholesterol to move more efficiently through the bloodstream.
Mutations in the lecithin cholesterol acyltransferase, or LCAT, gene cause familial LCAT deficiency, or FLD, a very rare metabolic disorder that affects the body’s ability to esterify cholesterol. LCAT is the only enzyme that esterifies cholesterol in plasma, while sterol O-acyltransferases 1 and 2, or SOAT1 and SOAT2, esterify cellular cholesterol in cells.
Patients with FLD have severe high-density lipoprotein deficiency, hypertriglyceridemia, and an elevated ratio of unesterified to total cholesterol. However, recent patient studies have shown that despite the absence of LCAT activity, carriers have circulating cholesterol esters or CEs and their plasma levels are highly variable.
Chiara Pavanello from the Università degli Studi di Milano and a team of researchers evaluated the origin of circulating CEs in plasma samples from carriers of LCAT deficiency. To their surprise, they found that, in the absence of LCAT-derived CEs, CEs are present in apoB-containing lipoproteins derived from liver and gut SOAT2. This work is described in a recent publication in the Journal of Lipid Research.
This is important because other studies have shown that the types of CEs that predominate in plasma contribute to the relative degree of atherogenicity (formation of abnormal fat or lipid masses in arterial walls). For example, the thickness of the carotid artery intima media is positively associated with the amount of circulating SOAT2-derived CEs, independent of other cardiovascular risk factors.