Haptoglobin inhibits phospholipid transfer protein activity in hyperlipidemic human plasma.

BACKGROUND: Haptoglobin is a plasma protein that scavenges haemoglobin during haemolysis. Phospholipid Transfer Protein (PLTP) transfers lipids from Low Density Lipoproteins (LDL) to High Density Lipoproteins (HDL). PLTP is involved in the pathogenesis of atherosclerosis which causes coronary artery disease, the leading cause of death in North America. It has been shown that Apolipoprotein-A1 (Apo-A1) binds and regulates PLTP activity. Haptoglobin can also bind to Apo-A1, affecting the ability of Apo-A1 to induce enzymatic activities. Thus we hypothesize that haptoglobin inhibits PLTP activity. This work tested the effect of Haptoglobin and Apo-A1 addition on PLTP activity in human plasma samples. The results will contribute to our understanding of the role of haptoglobin on modulating reverse cholesterol transport. RESULTS: We analyzed the PLTP activity and Apo-A1 and Haptoglobin content in six hyperlipidemic and six normolipidemic plasmas. We found that Apo-A1 levels are proportional to PLTP activity in hyperlipidemic (R2 = 0.66, p < 0.05) but not in normolipidemic human plasma. Haptoglobin levels and PLTP activity are inversely proportional in hyperlipidemic plasmas (R2 = 0.57, p > 0.05). When the PLTP activity was graphed versus the Hp/Apo-A1 ratio in hyperlipidemic plasma there was a significant correlation (R2 = 0.69, p < 0.05) suggesting that PLTP activity is affected by the combined effect of Apo-A1 and haptoglobin. When haptoglobin was added to individual hyperlipidemic plasma samples there was a dose dependent decrease in PLTP activity. In these samples we also found a negative correlation (-0.59, p < 0.05) between PLTP activity and Hp/Apo-A1. When we added an amount of haptoglobin equivalent to 100% of the basal levels, we found a 64 +/- 23% decrease (p < 0.05) in PLTP activity compared to basal PLTP activity. We tested the hypothesis that additional Apo-A1 would induce PLTP activity. Interestingly we found a dose dependent decrease in PLTP activity upon Apo-A1 addition. When both Apo-A1 and Hpt were added to the plasma samples there was no further reduction in PLTP activity suggesting that they act through a common pathway. CONCLUSION: These findings suggest an inhibitory effect of Haptoglobin over PLTP activity in hyperlipidemic plasma that may contribute to the regulation of reverse cholesterol transport.

Differences in human phospholipid transfer protein activity following incubation of Fungizone compared to lipid-based Amphotericin-B formulations in normolipidemic and hyperlipidemic plasma.

AIM: To investigate how different formulations of Amphotericin-B (Amp-B) affect the activity of phospholipid transfer protein (PLTP) when incubated with hyperlipidemic and normolipidemic plasma at physiological temperature (37 degrees C). METHODS: Six hyperlipidemic and six normolipidemic plasma samples were collected and tested for protein concentration. Equivalent protein levels (25 microg) were then tested for PLTP activity using an in vitro established kit at physiological temperature (37 degrees C). Increasing concentrations of different Amp-B formulations (1, 2, and 5 microg/mL) in the pharmacological range were then added to the plasma and tested for activity from 5 to 90 minutes. The Amp-B formulations used in the study were Fungizone, Abelcet, and AmBisome. RESULTS: In normolipidemic plasma, PLTP activity was found to be increased by Abelcet and AmBisome but inhibited by Fungizone. In hyperlipidemic plasma, PLTP activity was found to be increased by Abelcet and AmBisome but not changed by Fungizone. The Vm value for Abelcet and AmBisome was higher than Fungizone(; although, no difference was observed in the Km values between formulations. CONCLUSIONS: Findings suggest that lipid-based formulations of Amp-B promote the transfer of Amp-B into high-density lipoprotein fractions at a degree of increase inversely proportional to the lipid levels in the plasma.