Proteins

Composed of carbon, hydrogen, oxygen and nitrogen
Most abundant macromolecule in the body
Amphoteric in nature
Synthesized in the liver except for immunoglobulins (which are synthesized by plasma cells)
In alkaline Ph = proteins are negatively charged
In acidic pH = proteins are positively charged
Structures:
- Primary: amino acid sequence
- Secondary: conformations could either be alpha-helix, beta-pleated, sheath and bend form
- Tertiary: actual 3D configuration
- Quaternary: protein already consists of 2 or more polypeptide chains

PLASMA PROTEINS

FRACTIONS	SPECIFIC PROTEINS
Prealbumin	Aka transthyretin Marker for malnutrition 2nd most predominant protein in the CSF Transfer T4 and retinol (Vitamin A) Alcoholism, Chronic renal failure, steroid txm. poor nutrition RV: 18 – 45 mg/ dL
Albumin	Most abundant protein Acts as a transport protein Negative acute phase reactant Maintains osmotic pressure Elevated in Cystic fibrosis Negative acute phase reactant Low level: nephrotic syndrome Analbuminemia: albumin absence Bisalbuminemia: there are 2 bands seen in the albumin region Hypoalbuminemia: low levels of albumin RV: 3.5 – 5.0 g/dL
GLOBULIN	Measurement: TP – A = G Early cirrhosis RV: 2.3 – 3.5 g/dL
Alpha₁ globulin	Alpha₁ antitrypsin (AAT) Acute phase reactant. Released from WBC to combat inf Protease inhibitor NV 2.3-3.5 mg/dL AFP Tumor marker for hepatocellular carcinoma (hepatic and gonodal cancer). Increased in presence of twins and neural tube defect. Decreased in down syndrome. Screening for maternal AFP for NTD and DS: 15 and 20 weeks of gestational age. RV: 5 ng/ml both in adults and children Alpha-1-acid-glycoprotein (orosomucoid) Carrier proteins for steroid hormones (Progesterone). Increased in neonatal bacterial inf. RV: 55-140 mg/dL Alpha₁-antichymotrypsin Inhibits serine Proteinases Acute phase reactant. Binds and inactivates PSA Associated with Alzheimer’s dxs, in liver dxs RV: 30 – 60 mg/dL Gc-globulin Affinity with vit D and actin. Alpha-1-lipoprotein Transports lipids
Alpha₂ globulin	Ceruloplasmin Transports copper Wilson’s Disease (kayser-fleisher rings: deposition in cornea) Menkes’ kinky-hair syndrome Method: copper oxidase activity. RV: 18-45 mg/dL Haptoglobin transports free hemoglobin Acute phase reactant Alpha₂ macroglobulin Inhibits protease 10x elevation is seen in nephrosis RV: 150-420 mg/dL
Beta globulin	Pre-beta-lipoprotein Transports lipids (VLDL, TAG) Beta-lipoprotein Transports lipids (LDL, CHOLE) Beta₂ microglobulin Light chain component of HLA Elevated in RA and SLE, MM, HIV and Renal Failure. RV: 0.2-2.8 ug/dL Complement System Immune response increased in inflammation, decreased in DIC, hemolytic anemia and malnutrition CRP Acute phase reactants Promotes phagocytosis Cardiac marker RV: <1.0 mg/dL Fibrinogen Protein present in plasma but not in serum Largest protein in the blood Precursor of fibrin clot Method for measurement: Parfentjev method RV: <1.0 Hemopexin Acute phase reactant Binds heme Indicates early hemolysis RV: 50-115 mg/dL Transferrin/SIderophilin Transports iron hemochromatosis, liver dxs, malnutrition, nephrotic syn.
Gamma globulin	Immunoglobulins: synthesized in plasma cells IgG most abundant IgA found in mucous secretions IgM first to appear IgE allergy and anaphylactic reactions IgD present in surface of B cells CRP (in other references)

OTHERS PROTEINS

Myoglobin
- Carries oxygen in muscles
- Nephrotoxin
- Marker of chest pain (angina) and early det. Of AMI
- 2-3 hours of onset, peak at 8-10 hours
- AMI, angina, rhabdomyolysis, muscle trauma, acute renal failure
Troponin (cTnI)
- Regulates actin and myosin
- Marker for acute coronary syndrome
- Most important marker for AMI
- RV: <0.1 ng/mL
BNP
- ventricular systolic and diastolic dysfunction
- Congestive heart failure
Cystatin C
- Marker for kidney function (GFR)
- Endogenous renal marker
Beta-trace protein
- Marker for CSF leakage
Amyloid
- Fibrous protein aggregates
Bence-Jones protein: protein found in patients with Multiple Myeloma
- Unique feature: Coagulates at 40-60°C and dissolves at 100°C
- Method for measurement: Immunofixation
- Electrophoretic pattern: “tall spike” or “monoclonal peak”

METHODS FOR ALBUMIN QUANTITATION

Electrophoresis
Biuret Method
- Principle: measurement of at least 2 peptide bonds and formation of a violet colored chelate.
- Measured at 540nm
- Reagents: Rochelle salt (NaK tartrate), Alkaline CuSO₄, NaOH and KI
Kjeldahl Method
- Reference method
- Based upon the digestion of protein and measurement of nitrogen content of proteins
- Albumin nitrogen x 6.25 = albumin
Lowry (Folin-Ciocalteu) method
- Reagent: Phosphotungstomolybdic acid
Dye-binding method
- BCG: most commonly used
- BCP: most sensitive, specific and precise
- H-ABA: with salicylates and bilirubin interferences

CSF OLIGOCLONAL BANDING

Multiple sclerosis: 2 or more IgG bands in the gamma region
Other dxs with two more bands in the CSF: Encephalitis, neurosyphilis, Gullain-Barre syndrome, neoplastic dxs
Serum banding in CSF: Leukemia, lymphoma and viral inf.

AMINOACIDOPATHIES

Alkaptonuria
- Absence of homogentisate oxidase in tyrosine pathway
- Ochronosis: tissue pigmentation
- Darkening of urine upon standing
Homocystinuria
- Impaired activity of cystathionine B-synthetase
- Elevated homocysteine and methionine in blood and urine
- Screening test: Modified Guthrie Test (L-methionine sulfoximine)
MSUD
- Reduced or absence of a-ketoacid decarboxylase
- Accumulation of leucine, isoleucine and valine.
- Screening test: Modified Guthrie Test (4-azaleucine)
Phenylketonuria
- Def of phenylalanie hydrolase
- Phenylpyruvic acid in both blood and urine
- Musty odor urine
- Screening: Guthrie Bacterial Inhibition Assay (Bacillus subtillis)
Tyrosinemia
- of either of these enzymes tyrosine aminotransferase, 4-hydroxyphenylpyruvic acid oxidase, fumarylacetoacetate
- Increased levels of methionine and p-hydroxyphenolpyruvic acid in blood.
- Results to liver damage or cirrhosis

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Clinical Chemistry Notes

Proteins