Temperature-dependent immunoreactive assay to screen for digoxin-like immunoreactive factor(s).

Area of Science:

• Biochemistry
• Endocrinology
• Analytical Chemistry

Background:

• Endogenous digoxin-like immunoreactive factors (DLIF) cross-react with digoxin antibodies and inhibit Na+/K(+)-transporting ATPase (Na+K+ATPase).
• Human cord serum DLIF levels decrease with increased immunoassay temperature (4°C to 37°C).

Purpose of the Study:

• To investigate various compounds for cross-reactivity with digoxin immunoassays.
• To assess their ability to inhibit Na+K+ATPase.
• To determine if they exhibit temperature-dependent behavior similar to human cord serum DLIF.

Main Methods:

• Testing hormonal steroids, bile salts, lipids, and methionine-enkephalin.
• Utilizing two commercial 125I digoxin radioimmunoassays (RIAs).
• Assessing inhibition of porcine Na+K+ATPase activity at varying incubation temperatures (4°C vs. 37°C).

Main Results:

• Progesterone showed the highest cross-reactivity in digoxin RIAs.
• Most cross-reacting steroids, bile salts, and linoleic acid exhibited decreased immunoreactivity at 37°C compared to 4°C.
• Lysophosphatidyl lipids showed increased apparent digoxin concentration with higher incubation temperatures.

Conclusions:

• Multiple compounds can exhibit digoxin-like immunoreactivity and Na+K+ATPase inhibition.
• Temperature dependence of immunoreactivity may serve as a criterion for identifying true DLIF.
• This finding aids in distinguishing endogenous factors from assay interference.