Analysis of Lipoprotein(a) Catabolism

Elevated plasma concentrations of lipoprotein(a) [Lp(a)] have been identified as an independent risk factor for vascular diseases including coronary heart disease and stroke. In the current study, we have examined the binding and degradation of recombinant forms of apolipoprotein(a) [r-apo(a)], the unique kringle-containing moiety of Lp(a), using a cultured cell model. We found that the incubation of human hepatoma (HepG2) cells with an iodinated 17 kringle-containing (17K) recombinant form of apo(a) resulted in a two-component binding system characterized by a high affinity (Kd = 12 nM), low capacity binding site, and a low affinity (Kd = 249 nM), high capacity binding site. We subsequently determined that the high affinity binding site on HepG2 cells corresponds to the LDL receptor. In the HepG2 cell model, association of apo(a) with the LDL receptor was shown to be dependent on the formation of Lp(a) particles from endogenous LDL. Using an apo(a) mutant incapable of binding to the high affinity site through its inability to form Lp(a) particles (17KΔLBS7,8), we further demonstrated that the LDL receptor does not participate in Lp(a) catabolism. The low affinity binding component observed on HepG2 cells, familial hypercholesterolemia (FH) fibroblasts and human embryonic kidney (HEK) 293 cells may correspond to a member(s) of the plasminogen receptor family, as binding to this site(s) was decreased by the addition of the lysine analogue epsilon-aminocaproic acid. The lysine-dependent nature of the low affinity binding site was further confirmed in HepG2 binding studies utilizing r-apo(a) species with impaired lysine binding ability. We observed a reduction maximum binding capacity for 17K r-apo(a) variants lacking the strong lysine binding site (LBS) in kringle IV type 10 (17KΔAsp) and the very weak LBS in kringle V (17KΔV). Degradation of Lp(a)/apo(a) was found to be mediated exclusively by the low affinity component on both HepG2 cells and FH fibroblasts. Fluorescence confocal microscopy, using the 17K r-apo(a) variant fused to green fluorescent protein, further confirmed that degradation by the low affinity component on HepG2 cells does not proceed by the activity of cellular lysosomes. Taken together, these data suggest a potentially significant route for Lp(a)/apo(a) clearance in vivo.

Evaluating Spirulina as a protein source in Nile Tilapia (Oreochromis niloticus) grow-out diets

Aquaculture growth has intensified the need for a diversification of nutritionally appropriate aquafeed ingredients. The purpose of this study was to evaluate Spirulina, a blue-green microalgae, and soybean meal as the sole protein sources in grow-out Tilapia diets. We constructed 3 experimental diets with soybean meal and 0,15, 30, and 45% Spirulina (SBM, SP15, SP30, and SP45 respectively) as their main protein sources. We compared these diets to a commercial Tilapia diet (CC). Additionally, to evaluate the benefit of fishmeal inclusion, fishmeal was added (2 and 10%) to the most successful Spirulina containing diet (FM2, FM10). We evaluated these experimental diets based on their physical properties, palatability, growth potential, waste production, and overall cost. No significant differences in growth performance were found between any of the diets. Total ammonia nitrogen (TAN) and total phosphorus (TP) levels in each tank were significantly affected by diet (p<0.05). CC had significantly higher TP than the experimental diets and SP15 had significantly higher TAN than the other diets. Only CC was found to be significantly more palatable than the experimental diets, and Spirulina inclusion was inversely correlated to pellet stability. Lastly, SP15 was the most profitable experimental diet. We recommend eliminating fishmeal from grow-out Tilapia diets in favour of soybean meal and Spirulina. Spirulina should, however, be limited to 15% to avoid the negative effects it has on stability and profitability, and its possible effect on feed intake.