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Paediatric Laboratory Medicine

Complex II+III

Clinical Significance

To determine the rate of SCR in mitochondria as a result of mitochondrial respiratory complexes II and III activity.

Test Name

Complex II+III

Alternate Name/ Synonym

  • Succinate Cytochrome C Reductase

Gene Name

  • Caused by mutations in nuclear genes and mitochondrial genes


Mitochondrial Disease


Spectrophotometric Measurements at 550nm

Turn Around Time

  • Fibroblasts: 3 weeks - 3 months 
  • Tissue Biopsies: Fresh - 5 business days
  • Tissue Biopsies: Frozen - 1 month


  • Systemic Mitochondrial Syndromes
  • Leigh's

Specimen Type

  • Endocardial biopsy
  • Fibroblast extract and mitochondria
  • Muscle mitochondria 
  • Kidney, liver, muscle homogenates

Minimum Specimen Requirements

  • 1 dish for subculturing
  • 20 dishes for mitochondrial isolation from fibroblasts
  • 30-50 mg from muscle biopsy/autopsy for testing on muscle homogenate
  • 250-300 mg for mitochondrial isolation from fresh muscle biopsy

Optimal Collection Time

  • For fibroblasts, frozen biopsies: anytime
  • Fresh biopsy: until 2:30 pm,  Monday-Friday


  • For fibroblasts: Room temperature
  • Frozen biopsy: transport on dry ice
  • For fresh muscle biopsy: transport from Pathology in dish on ice

Special Requirements

For fresh biopsy: call to book (ext. 205989) at least 24 hours in advance for full Mitochondrial Enzyme screening on isolated mitochondria. Pick up from Pathology.

Shipping and Contact Information

The Hospital for Sick Children
Rapid Response Laboratory
170 Elizabeth Street, Room 3642
Toronto, ON
M5G 2G3
Phone: 416-813-7200
Phone: 1-855-381-3212

Reference Range

  • Fibroblast lysates: 3.89-15.67 nmol/mg/min
  • Fibroblast mitochondria: 7.40-51.07 nmol/mg/min, CII+III/CS: 0.20-0.79
  • Muscle mitochondria: 51.17-288.07 nmol/mg/min
  • Muscle homogenate: 0.67-3.03 µmol/min/g wet wt
  • Kidney homogenate: 0.68-2.37 µmol/min/g wet wt
  • Liver homogenate: 0.65-2.44 µmol/min/g wet wt
  • Endocardial homogenate: 52.99-165.09 nmol/mg/min

All rates reported in combination with CS rates.


Complex II performs a key step in the citric acid cycle, in which succinate is dehydrogenated to ubiquinone in the mitochondrial inner membrane. Complex III catalyzes electron transfer between ubiquinol and cytochrome C and also translocates protons across the mitochondrial inner membrane. This assay measures the combined activity of complex II and III.