Apolipoprotein B metabolism and the distribution of VLDL and LDL subfractions.

Apolipoprotein B (apoB) metabolism was investigated in 20 men with plasma triglyceride 0.66-2.40 mmol/l and plasma cholesterol 3.95-6. 95 mmol/l. Kinetics of VLDL(1) (S(f) 60-400), VLDL(2) (S(f) 20-60), IDL (S(f) 12-20), and LDL (S(f) 0;-12) apoB were analyzed using a trideuterated leucine tracer and a multicompartmental model which allowed input into each fraction. VLDL(1) apoB production varied widely (from 5.4 to 26.6 mg/kg/d) as did VLDL(2) apoB production (from 0.18 to 8.4 mg/kg/d) but the two were not correlated. IDL plus LDL apoB direct production accounted for up to half of total apoB production and was inversely related to plasma triglyceride (r = -0.54, P = 0.009). Percent of direct apoB production into the IDL/LDL density range (r = 0.50, P < 0.02) was positively related to the LDL apoB fractional catabolic rate (FCR). Plasma triglyceride in these subjects was determined principally by VLDL(1) and VLDL(2) apoB fractional transfer rates (FTR), i.e., lipolysis. IDL apoB concentration was regulated mainly by the IDL to LDL FTR (r = -0.71, P < 0.0001). LDL apoB concentration correlated with VLDL(2) apoB production (r = 0.48, P = 0.018) and the LDL FCR (r = -0.77, P < 0. 001) but not with VLDL(1), IDL, or LDL apoB production. Subjects with predominantly small, dense LDL (pattern B) had lower VLDL(1) and VLDL(2) apoB FTRs, higher VLDL(2) apoB production, and a lower LDL apoB FCR than those with large LDL (pattern A). Thus, the metabolic conditions that favored appearance of small, dense LDL were diminished lipolysis of VLDL, resulting in a raised plasma triglyceride above the putative threshold of 1.5 mmol/l, and a prolonged residence time for LDL. This latter condition presumably permitted sufficient time for the processes of lipid exchange and lipolysis to generate small LDL particles.

Sensitive mass fragmentographic determination of acidic catecholamine metabolites in human body fluids.

A sensitive and simple method for the simultaneous quantitation of homovanillic acid and vanilmandelic acid in small samples (0.1-1.0 ml) of urine, serum, plasma and cerebrospinal fluid is described. The stable dimethylthiophosphinyl methyl ester derivatives are detected specifically by mass fragmentography using the respective deuterated compounds as internal standards. Gas chromatographic separation is performed on a fused-silica DB-1 capillary column combined with a cold injection system for large sample volumes. Linear response curves and a detection limit of 1 ng/ml are obtained. The method has been applied to the localization of pheochromocytoma by selective determination of venous vanilmandelic acid.

Highly sensitive method for the quantitation of homovanillic acid in cerebrospinal fluid.

Homovanillic acid in minute samples (50-100 microliters) of cerebrospinal fluid can be quantitated as its O-dimethylthiophosphinate methyl ester. The derivative is determined after glass capillary gas chromatography (GC) with high sensitivity by using a phosphorus-specific thermionic detector or by mass fragmentography, combined with a large sample volume split-splitless injector. The dimethylthiophosphinic esters show excellent stability against moisture and air. The precision of the overall procedure is 5.4% (GC) and 3.8% (GC-mass spectrometry). The method shows good linearity (r = 0.9999) over three orders of magnitude, from 500 pg to 500 ng. The lowest detectable concentration of homovanillic acid is 2-5 ng/ml. Concentrations of homovanillic acid determined in cerebrospinal fluid were 9.9-63.1 ng/ml (n = 10, mean = 30.9 ng/ml).

Quantitation of adrenaline and noradrenaline from human plasma by combined gas chromatography--high-resolution mass fragmentography.

A gas chromatographic--high-resolution mass fragmentographic method for the simultaneous determination of adrenaline and noradrenaline from human plasma is presented. The catecholamines are separted by adsorption on alumina and converted by a selective, two-step procedure to the corresponding N-trifluoroacetyl-N-trimethylsilyl derivatives. The benzylic fragment C16H31O3Si3 (m/e 355.1568) of these derivatives is detected at a mass spectrometric resolving power of 5000. This high resolution detection was necessary to differentiate this fragment from others with the same nominal mass of 355 originating from the biological matrix and/or the bleeding from column and septum.

Mass spectrometry of dimethylthiophosphinates of aromatic hydroxy compounds.

The mass spectra of 20 differently substituted dimethylthiophosphinic esters of aromatic hydroxy compounds are presented. Fragmentation routes were investigated using high resolution mass measurements, decoupled metastable determinations and deuterium labelling. All compounds exhibited abundant molecular ions and typical phosphorus-containing ions. Characteristic elimination processes strongly dependent upon the respective type of substitution were observed. Due to their high stability, their great ease of formation and their good gas chromatographic properties these new types of derivatives are of special interest for establishing gas chromatography mass spectrometry profiles of acidic catecholamine metabolites.