Amino Acids & Proteins: Aminoacidopathies, Disease, and Analysis

Vocabulary practice flashcards covering inborn errors of metabolism, total protein testing methods, individual serum globulins, and serum/CSF protein electrophoresis patterns.

Phenylketonuria (PKU)

An autosomal recessive inborn error of metabolism ($1:15{,}000$ births) caused by an absence of phenylalanine hydroxylase (PAH), leading to extreme elevation of phenylalanine, brain toxicity resulting in mental retardation and microcephaly, and a characteristic musty body odor.

Phenylalanine Hydroxylase (PAH)

The liver enzyme that converts phenylalanine to tyrosine; its congenital deficiency or dysfunction is responsible for classical phenylketonuria.

Tyrosinemia

An inborn error of metabolism marked by the excretion of tyrosine and its metabolites into urine due to enzyme deficiencies in the tyrosine degradation pathway, with Type I being the most severe form.

Maple Syrup Urine Disease (MSUD)

An autosomal recessive metabolic disorder ($1:150{,}000$ births) caused by absent or reduced activity of branched-chain alpha-ketoacid decarboxylase, blocking the catabolism of leucine, isoleucine, and valine, producing a maple syrup odor in urine, breath, and skin.

Total Protein Reference Interval (Serum/Plasma)

$6.5 - 8.3\,\text{g/dL}$, comprising the sum of albumin and all globulin fractions in circulation.

Biuret Method

The standard quantitative method for total protein where cupric ions ($\text{Cu}^{2+}$) in an alkaline solution chelate with at least two peptide bonds to yield a violet complex whose absorbance is directly proportional to the number of peptide bonds.

Kjeldahl Method

The historic reference method for total protein determination based on quantifying total nitrogen content; it has been replaced in routine clinical laboratories because it is labor-intensive with too many steps.

Albumin

The most abundant serum protein ($60 - 80\%$ of total protein, reference interval $3.4 - 5.0\,\text{g/dL}$), synthesized exclusively by the liver ($9 - 12\,\text{g/day}$) without bodily storage, responsible for approximately $80\%$ of plasma oncotic pressure and serving as a general transport vehicle.

Albumin/Globulin (A/G) Ratio

A mathematical index with a normal reference range of $1.5 - 2.5$, calculated by dividing serum albumin concentration by total globulin concentration (total protein minus albumin); a decreased or inverted ratio indicates hepatic or renal pathology.

Barbital Buffer ($\text{pH } 8.6$)

The standard electrophoresis buffer that gives all serum ampholyte proteins a net negative charge, enabling them to migrate toward the anode (positive electrode).

Pre-albumin (Transthyretin)

A rapid-turnover visceral protein that migrates anodal to albumin on agarose gel, binds and transports vitamin A (retinol) as well as thyroid hormones ($\text{T}_3$ and $\text{T}_4$), and serves as a sensitive indicator of acute nutritional/protein status.

Alpha1-Antitrypsin (AAT)

An acute phase reactant and the major protease inhibitor within the alpha-1 band that neutralizes neutrophil elastase; congenital deficiency allows uninhibited elastase to degrade alveolar elastin, causing early-onset emphysema ($20 - 30$ years of age) and neonatal cirrhosis.

Alpha1-Fetoprotein (AFP)

The primary fetal serum protein analogous to adult albumin, used clinically as an oncofetoprotein/tumor marker for hepatocellular and germ cell carcinomas, as well as a maternal screening marker that is increased in neural tube defects and decreased in Down syndrome.

Alpha1-Acid Glycoprotein (AAG / Orosomucoid)

An acute phase reactant produced by the liver that carries a net negative charge even in acidic solutions and functions clinically to bind basic lipophilic therapeutic medications, altering their distribution and half-life.

Alpha2-Macroglobulin

A high-molecular-weight protease inhibitor that regulates fibrinolysis by binding plasmin; due to its large physical size, it is retained by damaged glomeruli in nephrotic syndrome, resulting in a marked relative elevation on electrophoretograms.

Haptoglobin

An alpha-2 positive acute phase reactant that binds free hemoglobin dimers following intravascular hemolysis to prevent renal clearance and conserve iron; its levels are characteristically depleted in hemolytic anemias.

Ceruloplasmin

A copper-binding alpha-2 glycoprotein and acute phase reactant; congenital reduction or absence leads to systemic toxic accumulation of elemental copper in tissues, termed Wilson's disease.

Transferrin (Siderophilin)

The principal beta-globulin iron transporter that reversibly binds two ferric ($\text{Fe}^{3+}$) atoms per molecule; it acts as a negative acute phase reactant and shows elevated serum concentrations during iron deficiency anemia and pregnancy.

Beta2-Microglobulin

The light-chain subunit of HLA antigens on nucleated cells; it is cleared by glomerular filtration and $99\%$ reabsorbed and catabolized by proximal renal tubules, making its serum elevation an index of tubular dysfunction and renal allograft rejection.

C-Reactive Protein (CRP)

A sensitive positive acute phase reactant that migrates in the gamma region, binding phosphocholine on bacteria and fungi to activate complement and facilitate phagocytosis; high-sensitivity assays (hs-CRP) detect baseline vascular inflammation associated with atherosclerosis.

Bence-Jones Protein

Free monoclonal immunoglobulin kappa ($\kappa$) or lambda ($\lambda$) light chains produced in multiple myeloma that pass into urine and demonstrate the diagnostic physical property of precipitating at $40 - 60\,^\circ\text{C}$ and redissolving when heated above $80\,^\circ\text{C}$.

Beta-Gamma Bridging

An electrophoretic phenomenon characteristic of chronic liver disease (cirrhosis) caused by polyclonal increase in immunoglobulins (predominantly $\text{IgA}$) that migrate into the beta region, obliterating the normal valley between beta and gamma bands.

Oligoclonal Bands

Discrete bands in the gamma fraction of cerebrospinal fluid (CSF) protein electrophoresis that are not identifiable in corresponding patient serum, observed in approximately $95\%$ of individuals with Multiple Sclerosis.

Nephrotic Syndrome Electrophoresis Pattern

A serum protein electrophoresis pattern characterized by a marked decrease in the albumin peak and gamma globulins due to urinary wasting, alongside a prominent relative increase in the alpha-2 globulin band ($\alpha_2$-macroglobulin).

Cirrhosis Electrophoresis Pattern

A serum protein electrophoresis profile showing reduced albumin synthesis and prominent beta-gamma bridging, where expanding polyclonal immunoglobulin production merges the beta and gamma fractions.

Monoclonal Gammopathy Electrophoresis Pattern

A serum protein electrophoresis tracing featuring an intense, narrow paraprotein spike (M spike) restricted to the gamma region, produced by clonal proliferation of neoplastic plasma cells (as seen in multiple myeloma).

Acute Inflammation Electrophoresis Pattern

A serum protein electrophoresis pattern demonstrating prominent increases in alpha-1 (primarily $\alpha_1$-antitrypsin) and alpha-2 (primarily haptoglobin) bands, accompanied by normal or slightly decreased albumin and a decrease in transferrin.

CSF Oligoclonal Banding Pattern

Electrophoretic separation of cerebrospinal fluid showing two or more distinct immunoglobulin bands in the gamma region that are absent from patient serum, indicating intrathecal antibody production.

Waldenström's Macroglobulinemia

A clonal lymphoid proliferation producing excess monoclonal $\text{IgM}$ paraprotein, resulting in serum hyperviscosity, cryoglobulinemia, and a monoclonal gamma-region electrophoretic spike.