What Is PINK1 and Parkin? The Mitophagy Pathway Explained

PINK1 and Parkin are the two proteins your cells use to label a worn-out mitochondrion for recycling. Understanding how they work is the clearest way to understand what mitophagy actually is — and why a compound like urolithin A has drawn so much research interest.

Every one of your cells runs on mitochondria, the tiny structures that convert food and oxygen into usable energy. Like any hard-working machine, individual mitochondria wear out. When they do, the cell faces a choice: leave the damaged unit in place, where it leaks reactive molecules and drains resources, or dismantle it cleanly and reuse the parts. The second option is called mitophagy — the selective autophagy of mitochondria — and the best-understood way your cells carry it out is the PINK1–Parkin pathway. This guide explains that pathway in plain language, why it matters for healthy aging, and where urolithin A fits into the picture.

The quick version

PINK1 is a sensor. Parkin is a tag-maker. When a mitochondrion becomes damaged, PINK1 collects on its surface and switches Parkin on. Parkin then coats the damaged mitochondrion in a molecular tag called ubiquitin. That tag is a label the cell's recycling machinery recognizes, so the flagged mitochondrion gets wrapped in a membrane and delivered to the lysosome, the cell's disposal and salvage compartment, where it is broken down. The useful building blocks are recovered and the damaged unit is gone. That is mitophagy in a sentence.

Meet the two main players

PINK1 — the damage sensor

PINK1 (PTEN-induced kinase 1) is an enzyme that is constantly being made and constantly being destroyed. On a healthy mitochondrion, PINK1 is imported inside, quickly cut up, and cleared — so healthy mitochondria carry almost none of it. This is the elegant part: PINK1's own instability is the sensor. A healthy mitochondrion maintains an electrical charge across its inner membrane, called the membrane potential, which it uses to import proteins. When a mitochondrion is damaged and loses that charge (becomes "depolarized"), the normal import-and-destroy process stalls. PINK1 can no longer be cleared, so it accumulates on the outer surface of exactly the mitochondria that are in trouble. In other words, PINK1 builds up as a direct readout of damage.

Parkin — the tag-maker

Parkin is an E3 ubiquitin ligase — an enzyme whose job is to attach ubiquitin tags to other proteins. Normally Parkin floats in the cell's fluid in an inactive, folded-up state. Once PINK1 has piled up on a damaged mitochondrion, PINK1 does two things: it phosphorylates (adds a phosphate group to) ubiquitin, and it phosphorylates Parkin itself at a specific spot known as Serine 65. That double signal recruits Parkin to the mitochondrion and unlocks it. Activated Parkin then blankets the outer membrane with more ubiquitin, and PINK1 phosphorylates those tags too, creating a self-amplifying "feed-forward" loop that quickly marks the whole organelle for disposal.

Step by step: how the pathway clears a damaged mitochondrion

It helps to walk through the sequence as a chain of events:

1. Damage and depolarization

A mitochondrion sustains enough damage — from accumulated stress, misfolded proteins, or loss of function over time — that it can no longer hold its inner-membrane charge. This is the trigger.

2. PINK1 accumulates

Because the depolarized mitochondrion can't import and destroy PINK1 anymore, PINK1 stacks up on its outer surface. The sensor has fired.

3. Parkin is recruited and switched on

PINK1 phosphorylates ubiquitin and Parkin at Serine 65. Parkin unfolds, becomes active, and moves from the cell fluid onto the damaged mitochondrion.

4. The ubiquitin coat is built

Active Parkin attaches chains of ubiquitin to proteins on the outer membrane. PINK1 keeps phosphorylating them, and the feed-forward loop rapidly amplifies the signal until the organelle is densely labeled.

5. Autophagy receptors read the tag

Adaptor proteins — including p62/SQSTM1, optineurin and NDP52 — recognize the phospho-ubiquitin coat. They act as bridges, linking the tagged mitochondrion to a growing double membrane studded with a protein called LC3.

6. Engulfment and recycling

The membrane closes around the mitochondrion to form an autophagosome, which fuses with a lysosome. Enzymes inside break the mitochondrion down, and the amino acids, lipids and other components are released back into the cell to be reused. The damaged unit is gone; the raw materials remain.

PINK1 and Parkin aren't the only route

The PINK1–Parkin pathway is the most thoroughly mapped, but it is not the cell's only way to clear mitochondria. A second class of mechanisms is receptor-mediated: proteins such as BNIP3, NIX and FUNDC1 sit on the mitochondrial membrane and can bind LC3 directly, skipping the ubiquitin-coating step. These routes matter in particular tissues and situations — for example, NIX is central to clearing mitochondria from maturing red blood cells. Research into urolithin A in simple organisms has pointed to involvement of the Nrf2-family transcription factor (called SKN-1 in the worm C. elegans), which helps regulate both mitophagy genes and the making of new mitochondria. The honest summary is that mitophagy is a network, not a single switch, and which route dominates depends on the cell type and the kind of stress involved.

Why this matters for healthy aging

Mitophagy is essentially quality control. When it runs well, the mitochondrial pool stays "young" on average, because worn units are removed and replaced. Researchers studying the biology of aging have observed that the efficiency of this housekeeping tends to decline with age in many tissues, which may allow damaged mitochondria to accumulate. That is a correlation drawn largely from laboratory and animal work, not a proven cause of any human condition, and it is important not to overstate it. But it does explain why mitophagy has become such an active target: if the recycling system slows down, supporting it is a logical place to look. To be clear, a healthy mitochondrial pool depends most on the fundamentals — regular physical activity, which is one of the strongest known stimulators of both mitophagy and new mitochondria, along with sleep and a varied diet.

How urolithin A connects to the pathway — honestly labeled

Here is what the evidence actually shows, separated by strength. In preclinical research (worms and rodents — not proof in humans), the foundational 2016 work by Ryu and colleagues in Nature Medicine reported that urolithin A increased the expression of mitophagy genes, improved muscle function in aged animals, and extended lifespan in C. elegans. Later animal work connected it to the SIRT1–PGC-1α axis and higher cellular energy currency (ATP and NAD+).

In human trials, the picture is more measured. The first-in-human study (Andreux et al., 2019, Nature Metabolism) found urolithin A was safe and well tolerated up to 1,000 mg per day and produced a mitochondrial gene-expression signature in muscle — but this was a safety and biomarker study, not a test of strength or performance. A 2022 trial in middle-aged adults (Singh et al., Cell Reports Medicine) reported that in the 500 mg group, muscle proteomics were most enriched for the "Parkin-mediated ubiquitin and proteasomal systems" pathway, with increased Parkin Serine 65 phosphorylation — the very activation marker described above — while the 1,000 mg group showed increases in oxidative-phosphorylation enzymes. A 2022 trial in older adults (Liu, D'Amico et al., JAMA Network Open) reported improved muscle fatigue resistance. So the human data are consistent with the PINK1–Parkin axis being engaged, but the trials are small, some outcomes were not statistically significant, and researchers have not directly proven the mechanism in people. For the fuller evidence picture, see our summary of the clinical studies.

Related reading

To go deeper, start with our plain-English urolithin A guide, then explore what mitophagy is, how autophagy and mitophagy relate, and the role of PGC-1α in making new mitochondria. If you're comparing products, our roundup of the best urolithin A gummies covers dose, testing and value.

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. This guide is educational and is not a substitute for advice from a qualified healthcare provider.