  Nuclear spin hyperpolarization enables real-time observation of metabolism
and intermolecular interactions in vivo. 1-13C-Pyruvate is the leading
hyperpolarized tracer currently under evaluation in several clinical trials as
a promising molecular imaging agent. Still, the quest for a simple, fast, and
efficient hyperpolarization technique is ongoing. Here, we describe that
continuous, weak irradiation in the audio-frequency range of the 13C spin at
121 {\mu}T magnetic field (\sim twiceEarth\apos s field) enables spin order
transfer from parahydrogen to 13C magnetization of 1-13C-pyruvate. These
so-called LIGHT-SABRE pulses couple nuclear spin states of parahydrogen and
pyruvate via the J-coupling network of reversibly exchanging Ir-complexes.
Using \sim 100% parahydrogen at ambient pressure, we polarized 51 mM of
1-13C-pyruvate in the presence of 5.1 mM Ir-complex continuously and repeatedly
to a polarization of 1.1% averaged over free and catalyst-bound pyruvate. The
experiments were conducted at -8{\deg}C), where almost exclusively bound
pyruvate was observed, corresponding to an estimated 11% polarization on bound
pyruvate. The obtained hyperpolarization levels closely match those obtained
via SABRE-SHEATH under otherwise identical conditions. The creation of three
different types of spin orders was observed: transverse 13C magnetization along
the applied magnetic field, 13C z-magnetization along the main field B_0, and
13C-1H zz-spin-order. With a superconducting quantum interference device
(SQUID) for detection, we found that the generated spin orders result from tiny
1H-13C J-coupling interactions, which are not visible even with our narrow
linewidth below 0.3 Hz.
