Updated: 7/9/2019

Fatty Acid Metabolism

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Overview
  •  Structure
    • long chain of carbons with carboxyl group on one end
    • can have a variable amount of double bonds
      • double bonds make a fat unsaturated
      • naturally in a cis configuration
        • trans fats are unnatural and created via hydrogenation of vegetable oils
          • ↑ risk of atherosclerosis
      • double bonds ↓ melting temperature
        • plant fat (e.g. olive oil) is unsaturated and liquid at room temperature
        • animal fat (eg. butter) is saturated and solid at room temperature
    • nomenclature
      • e.g. palmitic acid
        • C16:0
          • 16 carbons with no double bonds
            • numbered with carboxyl carbon as 1
      • e.g. linoleic acid
        • C18:2 (9,12)
          • 18 carbons with 2 double bonds (one at the 9th and one at the 12th carbon)
      • omega system
        • count opposite to the numbered system (i.e. carboxyl carbon is counted last)
        • used to number unsaturated fats
        • e.g. linoleic acid
          • omega 6 family
          • double bond at position "12" is 6 in from the opposite side (18 carbons in total)
  • Essential fatty acids (FA)
    • cannot be synthesized
    • examples
      • linoleicacid
        • omega 6
        • can be used as a precursor for arachidonic acid
          • becomes an essential fatty acid if linoleic acid is absent
      • linolenic acid
        • omega 3
          • ↓ risk of CV disease
            • remember: omega 3 saves you from triple bypass
          • found in cold water fish, nuts
  • Transport
    • see Lipoprotein topic
Synthesis
  • FA synthesis
    • pyruvate (carbohydrate) → acetyl-CoA
      • activated by insulin
      • functions to store excess carbs as fat
      • occurs in the mitochondria via pyruvate dehydrogenase
    • acetyl-CoA + oxaloacetate → citrate
      • shuttled out of mitochondria into cytoplasm
        • citrate shuttle
      • split back to acetyl-CoA and oxaloacetate
    • acetyl-CoA + CO2→ malonyl-CoA
      • catalyzed by acetyl-CoA carboxylase
      • biotin required
      • activated by insulin
    • malonyl-CoA → CO2 + 2 carbons on fatty chain
      • catalyzed by FA synthase
      • requires NADPH
      • humans make palmitic acid (16:0) as stored fat
        • only de novo fat possible
      • for 1 palmitic acid requires
        • 8 acetyl-CoA
        • 7 ATP
        • 14 NADPH
Catabolism
  • Break down via β-oxidation
    • occurs in hepatocytes, myocytes, adipocytes
      • neurons cannot use fat as energy
        • FAs do not cross BBB
    • pathway location differs based on length of FAs
      • short/medium (2-12 carbons)
        • diffuse in mitochondria
      • long (14-20 carbons)
        • utilizes carnitine shuttle
          • carnitine added to FA in the intermembrane space of the mitochondria
            • catalyzed by carnitine acyltransferase (CAT) -1
              • inhibited by malonyl-CoA so as to prevent newly synthesized FAs from being degraded
          • carnitine: FA transported into the matrix
            • catalyzed by the carnitine transporter
          • carnitine exchanged for CoA
            • catalyzed by carnitine acyltransferase (CAT)-2
            • clinical importance
              • myopathic CAT deficiency
                • presentation
                  • myoglobinuria
                  • muscle aches/weakness
                  • ↑ TG content in muscles
                    • unable to use as energy
                  • provoked by prolonged use of muscle
      • very long (>20 carbons)
        • oxidized in peroxisome
  • β-oxidation pathway
    • occurs in the mitochondrial matrix
    • reverses FA synthesis
      • removing an acetyl-CoA and producing NADH and FADH2
        • catalyzed by fatty acyl-CoA dehydrogenase
          • two types
            • long-chain acyl-CoA dehydrogenase (LCAD)
            • medium-chain acyl-CoA dehydrogenase (MCAD)
          • blocked by ackee fruit toxin
      • creates most of the energy used by the liver
        • acetyl-CoA created in liver does not enter the citric acid cycle
          • forms ketones
            • see Ketone bodies topic
    • clinical importance
      • MCAD deficiency 
        • presentation 
          • non-ketotic hypoglycemia
          • C8-C10 acyl carnitines in the blood
            • liver unable to break FAs down further than C8-C10
          • no ketone bodies
            • liver unable to produce ketones from β-oxidation
          • fasting hypoglycemia
            • liver unable to produce enough energy from β-oxidation to supply gluconeogenesis
          • symptoms often precipitated by infection or stress
        • treatment
          • low fat diet with frequent meals of high carbs
 

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Questions (3)
Lab Values
Blood, Plasma, Serum Reference Range
ALT 8-20 U/L
Amylase, serum 25-125 U/L
AST 8-20 U/L
Bilirubin, serum (adult) Total // Direct 0.1-1.0 mg/dL // 0.0-0.3 mg/dL
Calcium, serum (Ca2+) 8.4-10.2 mg/dL
Cholesterol, serum Rec: < 200 mg/dL
Cortisol, serum 0800 h: 5-23 μg/dL //1600 h:
3-15 μg/dL
2000 h: ≤ 50% of 0800 h
Creatine kinase, serum Male: 25-90 U/L
Female: 10-70 U/L
Creatinine, serum 0.6-1.2 mg/dL
Electrolytes, serum  
Sodium (Na+) 136-145 mEq/L
Chloride (Cl-) 95-105 mEq/L
Potassium (K+) 3.5-5.0 mEq/L
Bicarbonate (HCO3-) 22-28 mEq/L
Magnesium (Mg2+) 1.5-2.0 mEq/L
Estriol, total, serum (in pregnancy)  
24-28 wks // 32-36 wks 30-170 ng/mL // 60-280 ng/mL
28-32 wk // 36-40 wks 40-220 ng/mL // 80-350 ng/mL
Ferritin, serum Male: 15-200 ng/mL
Female: 12-150 ng/mL
Follicle-stimulating hormone, serum/plasma Male: 4-25 mIU/mL
Female: premenopause: 4-30 mIU/mL
midcycle peak: 10-90 mIU/mL
postmenopause: 40-250
pH 7.35-7.45
PCO2 33-45 mmHg
PO2 75-105 mmHg
Glucose, serum Fasting: 70-110 mg/dL
2-h postprandial:<120 mg/dL
Growth hormone - arginine stimulation Fasting: <5 ng/mL
Provocative stimuli: > 7ng/mL
Immunoglobulins, serum  
IgA 76-390 mg/dL
IgE 0-380 IU/mL
IgG 650-1500 mg/dL
IgM 40-345 mg/dL
Iron 50-170 μg/dL
Lactate dehydrogenase, serum 45-90 U/L
Luteinizing hormone, serum/plasma Male: 6-23 mIU/mL
Female: follicular phase: 5-30 mIU/mL
midcycle: 75-150 mIU/mL
postmenopause 30-200 mIU/mL
Osmolality, serum 275-295 mOsmol/kd H2O
Parathyroid hormone, serume, N-terminal 230-630 pg/mL
Phosphatase (alkaline), serum (p-NPP at 30° C) 20-70 U/L
Phosphorus (inorganic), serum 3.0-4.5 mg/dL
Prolactin, serum (hPRL) < 20 ng/mL
Proteins, serum  
Total (recumbent) 6.0-7.8 g/dL
Albumin 3.5-5.5 g/dL
Globulin 2.3-3.5 g/dL
Thyroid-stimulating hormone, serum or plasma .5-5.0 μU/mL
Thyroidal iodine (123I) uptake 8%-30% of administered dose/24h
Thyroxine (T4), serum 5-12 μg/dL
Triglycerides, serum 35-160 mg/dL
Triiodothyronine (T3), serum (RIA) 115-190 ng/dL
Triiodothyronine (T3) resin uptake 25%-35%
Urea nitrogen, serum 7-18 mg/dL
Uric acid, serum 3.0-8.2 mg/dL
Hematologic Reference Range
Bleeding time 2-7 minutes
Erythrocyte count Male: 4.3-5.9 million/mm3
Female: 3.5-5.5 million mm3
Erythrocyte sedimentation rate (Westergren) Male: 0-15 mm/h
Female: 0-20 mm/h
Hematocrit Male: 41%-53%
Female: 36%-46%
Hemoglobin A1c ≤ 6 %
Hemoglobin, blood Male: 13.5-17.5 g/dL
Female: 12.0-16.0 g/dL
Hemoglobin, plasma 1-4 mg/dL
Leukocyte count and differential  
Leukocyte count 4,500-11,000/mm3
Segmented neutrophils 54%-62%
Bands 3%-5%
Eosinophils 1%-3%
Basophils 0%-0.75%
Lymphocytes 25%-33%
Monocytes 3%-7%
Mean corpuscular hemoglobin 25.4-34.6 pg/cell
Mean corpuscular hemoglobin concentration 31%-36% Hb/cell
Mean corpuscular volume 80-100 μm3
Partial thromboplastin time (activated) 25-40 seconds
Platelet count 150,000-400,000/mm3
Prothrombin time 11-15 seconds
Reticulocyte count 0.5%-1.5% of red cells
Thrombin time < 2 seconds deviation from control
Volume  
Plasma Male: 25-43 mL/kg
Female: 28-45 mL/kg
Red cell Male: 20-36 mL/kg
Female: 19-31 mL/kg
Cerebrospinal Fluid Reference Range
Cell count 0-5/mm3
Chloride 118-132 mEq/L
Gamma globulin 3%-12% total proteins
Glucose 40-70 mg/dL
Pressure 70-180 mm H2O
Proteins, total < 40 mg/dL
Sweat Reference Range
Chloride 0-35 mmol/L
Urine  
Calcium 100-300 mg/24 h
Chloride Varies with intake
Creatinine clearance Male: 97-137 mL/min
Female: 88-128 mL/min
Estriol, total (in pregnancy)  
30 wks 6-18 mg/24 h
35 wks 9-28 mg/24 h
40 wks 13-42 mg/24 h
17-Hydroxycorticosteroids Male: 3.0-10.0 mg/24 h
Female: 2.0-8.0 mg/24 h
17-Ketosteroids, total Male: 8-20 mg/24 h
Female: 6-15 mg/24 h
Osmolality 50-1400 mOsmol/kg H2O
Oxalate 8-40 μg/mL
Potassium Varies with diet
Proteins, total < 150 mg/24 h
Sodium Varies with diet
Uric acid Varies with diet
Body Mass Index (BMI) Adult: 19-25 kg/m2
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